Disable macOS builds on PRs while we wait for Travis to catch up. (#13389)

Travis macOS builds are taking too long to run, so we'll only run them on
`develop` until they get faster.

.travis.yml

@@ -56,6 +56,7 @@ jobs:
     - os: osx
       language: generic
       env: [ TEST_SUITE=unit, PYTHON_VERSION=2.7, COVERAGE=true ]
+      if: type != pull_request
 # mpich (AutotoolsPackage)
     - stage: 'build tests'
       python: '2.7'