Add xcodebuild step to GH Action to test all platform builds for non-SwiftPM workflows

[DivineDominion]

The current GitHub action only uses swift build and swift test. That's good enough to catch some errors, but it doesn't catch CocoaPods or Carthage build problems.

Adding an xcodebuild step and a test matrix for multiple platforms would be a good idea.

We can grab a working test matrix from ReSwift:
https://github.com/ReSwift/ReSwift/blob/master/.github/workflows/test.yml

[DivineDominion]

Might also update CONTRIBUTING.md to reflect that Travis CI is no more, and how things are actually run