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)

Run PCK specs using the following SDKs: 6.0, 6.1, 7.0, 7.1, all built with 7.1.