Resolves #4 `Update to Swift 3.1`

- Migrate to Swift 3.1
- Update to SwiftLint 0.19.0
- Update Travis to Xcode 8.3 image
- Update Dockerfile to `3.1` tag for base image (now that IBM is tagging releases)
- Update depdencies
- Resolve compiler warnings in test suite
- Work around `swift package generate-xcodeproj` not supporting the `--chdir` flag correctly
- Fix new linting issues (from `rubocop`)
- Use new `Package.pins` file to pin the dependency versions more accurately