Summary:
Add basic UI Testing to Examples-iOS app.
It doesn't cover all the examples yet, but Xcode is reporting ~82% of coverage.
Updated travis.yml to run the UI Tests for only one of the matrix jobs - "OS=10.0,name=iPhone 6s", to try to keep the build times acceptable. Last job took ~3 min for the UI Testing section (being the whole build usually around 40 min).
Was feeling like playing a bit with UI Testing and took the chance to use it here, since rnystrom [showed interest in having it](https://github.com/Instagram/IGListKit/pull/251#issuecomment-262375087).
- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/268
Differential Revision: D4259177
Pulled By: jessesquires
fbshipit-source-id: 77fdaf8588e090e4b13836a127d9f6d838248163
