[ios, build] Move iOS release builds to CircleCI

[friedbunny]

Fixes #11231. Moves our iOS release build process from Bitrise to CircleCI.

• Deletes the Bitrise config, as it’s no longer necessary.
• Pins this job to Ruby 2.3.x, as the jazzy gem had issues with stock Ruby 2.0.x.
  • This required a --login shell for chruby.
• Borrows @MapboxCI’s Danger GitHub token, until we can get this job one of its own.
• Cherry-picked [build] Update to Xcode 9.2 on CircleCI #10893, because we need to build releases with the latest stable Xcode.

/cc @mapbox/maps-ios @kkaefer