ci: change apt source on travis

clang 3.7 trusty version seems to be restricted. I don't know why, but
precise version seems to be working.

.travis.yml

@@ -70,7 +70,7 @@ matrix:
       apt:
         sources:
         - ubuntu-toolchain-r-test
-        - llvm-toolchain-trusty-3.7
+        - llvm-toolchain-precise-3.7
         - sourceline: 'ppa:mhier/libboost-latest'
         packages:
         - clang-3.7