[travis] fast_finish + allow_failures for OSX builds

Travis CI has been experiencing maintainance problems with the OSX infrastructure
and it has become too slow. It's better we temporarily enable 'allow_failures'
in combination with 'fast_finish' for the OSX builds, so that the CI goes green
as soon as the other builds have completed.

.travis.yml

@@ -2,6 +2,10 @@ sudo: false
 
 language: python
 
+# empty "env:" is needed for 'allow_failures'
+# https://docs.travis-ci.com/user/customizing-the-build/#Rows-that-are-Allowed-to-Fail
+env:
+
 matrix:
   fast_finish: true
   include:
@@ -32,6 +36,17 @@ matrix:
         - PYENV_VERSION_STRING='Python 2.7.6'
         - PYENV_ROOT=$HOME/.travis-pyenv
         - TRAVIS_PYENV_VERSION='0.4.0'
+  allow_failures:
+    # We use fast_finish + allow_failures because OSX builds take forever
+    # https://blog.travis-ci.com/2013-11-27-fast-finishing-builds
+    - language: generic
+      os: osx
+      env: TOXENV=py27-cov
+    - language: generic
+      os: osx
+      env:
+        - TOXENV=py36-cov
+        - HOMEBREW_NO_AUTO_UPDATE=1
 
 cache:
   - pip