Ignore macOS failures on Travis CI (#2345)

Since the Travis CI OS X infrastructure is incredibly unstable,
we need to let the builds run on a best effort basis,
but not report failure and not hang a result until its finished.

With this setup, the Linux build dictates the success,
completely ignoring OS X.

.travis.yml

@@ -10,6 +10,11 @@ sudo: required
 dist: trusty
 osx_image: xcode7.3
 
+matrix:
+  allow_failures:
+    - os: osx
+  fast_finish: true
+
 addons:
   artifacts:
     paths: $(ls powershell*{deb,pkg} | tr "\n" ":")