Update to Swift 5.1, version to 2.0.

[jshier]

This PR makes no logic changes, but instead updates the app for Swift 5.1 and other environment updates. This includes:

• A proper Ruby environment using .ruby-version and .ruby-gemset.
• More SwiftLint warnings and fixes.
• Updated dependencies.
• Updated Travis config.
• Versioning to 2.0.
• Update to Swift 5.1, including implicit returns.
• Deployment iOS version to 10.0.
• Removed executable flag from Gemfile, Podfile, and LICENSE.

[dtrenz]

Released in 2.0.0.