Use `.travis.yml` notifications for `.travis.osx.yml`

This copies over the notifications settings of the main
`.travis.yml` to `.travis.osx.yml` so that we can maintain
them centrally.

The change also tweaks the commit message to aid in quick
linking / correspondence of a pantsbuild/pants-for-travis-osx-ci
commit to a pantsbuild/pants commit.

Testing Done:
Ad hoc:
```bash
$ venv_setup pyyaml PyYAML && source ~/.pythons/venvs/pyyaml/bin/activate
(pyyaml)$ python << EOF
> import yaml
> with open('.travis.yml') as fp:
>   config = yaml.safe_load(fp)
> with open('.travis.osx.yml') as fp:
>   config_new = yaml.safe_load(fp)
> config_new['notifications'] = config['notifications']
> with open('.travis.new.yml', 'w') as fp:
>   yaml.safe_dump(config_new, fp)
> EOF
(pyyaml)$ cat .travis.new.yml
language: objective-c
notifications:
  email: [john.sirois@gmail.com, lhosken@twitter.com, zundel@squareup.com, benjyw@gmail.com,
    areitz@twitter.com]
script: 'sw_vers

  python --version

  java -version

  ./build-support/bin/ci.sh -d

  '
```

Bugs closed: 601

Reviewed at https://rbcommons.com/s/twitter/r/1067/
Sync of https://github.com/pantsbuild/pants/commit/d3266a814fcc72292c3e6b6803fc74f71f842469