Fix Travis builds

[kylefleming]

This PR does the following:

• Upgrades Travis's macOS image to use Xcode 10.2 to add support for Swift 4.2 and 5. This is because the latest version of Lottie (which is used in the tests) requires Swift 4.2 support. Xcode 10 adds support for Swift 4.2 and Xcode 10.2 adds support for Swift 5.
• Specifies RxCocoa 4.x in the tests. There appears to be incompatibilities somewhere in the tests with RxCocoa 5, at least on my machine, using Xcode 11.1. I believe sticking to RxCocoa 4.x will alleviate the issue.
• Fixes a number of issues with the Travis CI environment, including:
  • Setting the gemfile to test/Gemfile (so the correct gems are installed/used)
  • Allowing Bundler 2.x
  • Running pod setup (so the default repo is installed and updated)
  • Unsetting the CPATH environment variable (For some reason this variable is set by Travis, but is set to the macOS SDK which causes issues with xcodebuild).