Add a CI action to support CI against OSX.

This adds a sync action run on any push to pantsbuild/pants
master that replaces pantsbuild/pants-for-travis-osx-ci master
modifying the active .travis.yml to trigger an OSX CI run.

Testing Done:
Normal CI went green here: https://travis-ci.org/pantsbuild/pants/builds/35900516

OSX CI went green here: https://travis-ci.org/pantsbuild/pants-for-travis-osx-ci/builds/35899630
Via:
```console
$ set | grep GH_ | cut -d= -f1
GH_EMAIL
GH_TOKEN
GH_USER
$ ./build-support/bin/ci-sync.sh
[jsirois/pants-for-travis-osx-ci/sync 9a13170] Prepare pants OSX mirror for CI.
 Author: Pants CI <pants-ci@googlegroups.com>
 1 file changed, 13 insertions(+), 40 deletions(-)
 rewrite .travis.yml (87%)
Counting objects: 37, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (18/18), done.
Writing objects: 100% (23/23), 3.05 KiB | 0 bytes/s, done.
Total 23 (delta 14), reused 9 (delta 3)
To https://github.com/pantsbuild/pants-for-travis-osx-ci.git
 + 888c8bf...9a13170 HEAD -> master (forced update)
$ gitbinfo
* jsirois/pants-for-travis-osx-ci/sync 9a13170 [pants/master: ahead 2] Prepare pants OSX mirror for CI.
$ git slog -2
commit 9a13170039dfe93824fd9487f78f96c134e09d88
Author: Pants CI <pants-ci@googlegroups.com>
Date:   Sun Sep 21 20:22:52 2014 -0600

    Prepare pants OSX mirror for CI.

 .travis.yml | 35 ++++-------------------------------
 1 file changed, 4 insertions(+), 31 deletions(-)

commit f4cb72ba4cc48ec5f6a23c86903fe83b735a8f3f
Author: John Sirois <john.sirois@gmail.com>
Date:   Sun Sep 21 17:30:29 2014 -0600

    Add a CI action to support CI against OSX.
...
```

Bugs closed: 596

Reviewed at https://rbcommons.com/s/twitter/r/1058/