Split up tests better in travis

[nickmshelley]

Currently Travis runs two jobs, an iOS one and a Mac one. The majority of time spent on both of these jobs is in the cocoapods tests, which test all platforms (iOS, tvOS, watchOS, and macOS) in both jobs.

I think ideally there would be a job or two for unit tests, and then probably a cocoapods test job for each platform. This would keep us from running into the 50 minute Travis time limit for Mac jobs.