[travis-ci] Do automatic builds via Travis-CI on each commit and Pull Request

For example: https://travis-ci.org/tsnow/gitx/builds/31249519
Note that this does not currently push the build artifacts anywhere, it just runs the build and any available tests to detect errors.

Let me know if you'd like it to work with multiple [SDKs](http://docs.travis-ci.com/user/languages/objective-c/#Supported-iOS-SDK-versions), or if you'd like help [setting up a public travis-ci.org account](http://docs.travis-ci.com/user/getting-started/#Step-one%3A-Sign-in) and linking your repo.



#### Todo:

- [ ] Any changes necessary from PR comments.
- [ ] Sign off by @rowanj.
- [ ] Squash commits.
- [ ] rowanj/gitx turned on and linked at travis-ci.org. 
- [ ] Merged.