message: Travis: Stop testing on OS X 10.11.
  It continuously fails now:
  /Users/travis/.travis/job_stages: line 104: pip3: command not found
  While it's possible to fix this error, we still have newer macs in
  Travis.
diff: @@ -33,10 +33,6 @@ matrix:
  - os: linux
  compiler: clang
  env: BUILD_TYPE=tsan BUILD_WITH=cmake COVERAGE=no
  - - os: osx
  - osx_image: xcode7.3 # OS X 10.11
  - compiler: clang
  - env: BUILD_TYPE=normal BUILD_WITH=cmake COVERAGE=llvm
  - os: osx
  osx_image: xcode8.3 # macOS 10.12
  compiler: clang
  