Disable Travis CI OS X support

Unfortunately, Travis CI's OS X machines are 10.9.5, per
https://docs.travis-ci.com/user/osx-ci-environment/#CI-environment-OS.
While NuGet supports the RID, .NET Core does not yet publish 10.9
packages, resulting in a build failure we can't do anything about.

I've asked for best-effort support by providing runtime packages for
10.9 in dotnet/corefx#7731.

Revert "Add OS X 10.9 runtime"

This reverts commit 19ab0de84cec37bb90443df14d4e69f10c41e1b1.

.travis.yml

@@ -4,9 +4,9 @@ matrix:
     - os: linux
       dist: trusty
       sudo: required
-    - os: osx
-      language: generic
-      env: PATH+=/usr/local/share/dotnet
+    # - os: osx
+    #   language: generic
+    #   env: PATH+=/usr/local/share/dotnet
 git:
   submodules: false
 before_install: