diff --git a/.travis.yml b/.travis.yml
index 0ddf2e8..3542fc3 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -9,7 +9,7 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
         packages:
         - g++-5
@@ -21,7 +21,7 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
         packages:
         - g++-6
@@ -33,7 +33,7 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
         packages:
         - g++-7
@@ -45,7 +45,7 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
         packages:
         - g++-8
@@ -57,7 +57,7 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
         packages:
         - g++-8
@@ -69,7 +69,7 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
         packages:
         - g++-8
@@ -81,7 +81,7 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
         packages:
         - g++-8
@@ -89,16 +89,16 @@ matrix:
   - os: linux
     language: cpp
     compiler: clang
-    env: COMPILER="clang++-3.7" CXX_STANDARD=11 TOML_HEAD=OFF
+    env: COMPILER="clang++-3.9" CXX_STANDARD=11 TOML_HEAD=OFF
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
-        - llvm-toolchain-precise-3.7
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-3.9
         packages:
         - g++-8
-        - clang-3.7
+        - clang-3.9
         - boost1.70
   - os: linux
     language: cpp
@@ -107,9 +107,9 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
-        - llvm-toolchain-trusty-4.0
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-4.0
         packages:
         - g++-8
         - clang-4.0
@@ -121,9 +121,9 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
-        - llvm-toolchain-trusty-5.0
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-5.0
         packages:
         - g++-8
         - clang-5.0
@@ -135,9 +135,9 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
-        - llvm-toolchain-trusty-6.0
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-6.0
         packages:
         - g++-8
         - clang-6.0
@@ -149,9 +149,9 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
-        - llvm-toolchain-trusty-7
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-7
         packages:
         - g++-8
         - clang-7
@@ -163,9 +163,9 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
-        - llvm-toolchain-trusty-8
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-8
         packages:
         - g++-8
         - clang-8
@@ -177,9 +177,9 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
-        - llvm-toolchain-trusty-8
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-8
         packages:
         - g++-8
         - clang-8
@@ -191,9 +191,9 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
-        - llvm-toolchain-trusty-8
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-8
         packages:
         - clang-8
         - g++-8
@@ -205,9 +205,9 @@ matrix:
     addons:
       apt:
         sources:
-        - ubuntu-toolchain-r-test
-        - llvm-toolchain-trusty-8
+        - sourceline: 'ppa:ubuntu-toolchain-r/test'
         - sourceline: 'ppa:mhier/libboost-latest'
+        - llvm-toolchain-trusty-8
         packages:
         - clang-8
         - g++-8
@@ -216,6 +216,15 @@ matrix:
     language: cpp
     compiler: clang
     env: CXX_STANDARD=11
+    cache:
+      directories:
+        - $HOME/Library/Caches/Homebrew
+    addons:
+      homebrew:
+        update: true
+        packages:
+        - cmake
+        - boost
 
 script:
 - |
@@ -224,8 +233,6 @@ script:
     travis_retry wget "https://github.com/Kitware/CMake/releases/download/v3.14.5/cmake-3.14.5-Linux-x86_64.tar.gz"
     tar xf cmake-3.14.5-Linux-x86_64.tar.gz -C cmake --strip-components=1
     export PATH=${TRAVIS_BUILD_DIR}/cmake/bin:${PATH}
-  else
-    brew upgrade cmake boost
   fi
 - cmake --version
 - mkdir build
@@ -234,3 +241,10 @@ script:
 - cmake -DCMAKE_CXX_COMPILER=$COMPILER -DCMAKE_CXX_STANDARD=$CXX_STANDARD -DTOML11_USE_UNRELEASED_TOML_FEATURES=${TOML_HEAD} ..
 - make
 - ctest --output-on-failure
+
+# https://stackoverflow.com/a/53331571
+before_cache:
+- |
+  if [[ "${TRAVIS_OS_NAME}" == "osx" ]]; then
+    brew cleanup
+  fi