Support Swift 5

[mindgraffiti]

Closes #200.

This PR:

1. Updates pods to support Swift 5
2. Make Xcode recommended changes to support Swift 5
3. Updates the project file to remove a warning.

Changes made by Platform 9 3/4: updated tooling to support Xcode 10.2 and Swift 5 🌟

First reviewer wins! 🏆

To test

• bundle exec pod install ensure no warnings are spit out in the command line
• build the framework
• run the unit tests
• no warnings should appear in Xcode

To test in a host app

• check out this PR for WCiOS as the host app: https://github.com/woocommerce/woocommerce-ios/pull/924
• bundle exec pod install. No warnings should appear in the command line.
• run unit tests
• run app

[mindgraffiti]

Sorry about the early request to review. It's not ready yet.

[mindgraffiti]

I disabled my Chrome Javascript blocker. It's doing weird things when I request reviewers.

[mindgraffiti]

Thanks @astralbodies!