Run CI on iOS 6.0, 6.1, 7.0, and 7.1

[travis CI should run against iOSes 6, 6.1, 7, 7.1](https://www.pivotaltracker.com/story/show/67130024)

This runs all combinations (16) of the Cedar SDK against the Cedar runtime using iOS 6.0, 6.1, 7.0, and 7.0.