Fixes #12 - The project is not compatible with Xcode 12

[st3fan]

This patch brings this project up to date with Xcode 12 and Swift 5. The changes were pretty trivial, mostly some deprecations and new names. The iOS deployment target has not changed, it is still iOS 12.

This cannot be merged just yet as it would break the current [mozilla-mobile/focus-ios] project, which is still being built with Xcode 10. Let's keep this PR around until it is time to bring Focus iOS up to date.

cc @boek