ci: Cache cocoapods on CI (#290)
ci: Cache cocoapods on CI (#290)

## Goal

Caches cocoapods on CI, which should reduce the amount of time required for jobs to run. This is recommended on the [Travis CI docs](https://docs.travis-ci.com/user/caching/#CocoaPods).

bugsnag/bugsnag-cocoa,bugsnag/bugsnag-cocoa,bugsnag/bugsnag-cocoa,bugsnag/bugsnag-cocoa