Create a setup_gitlab action for Fastlane

Some other CI platforms have custom setup actions built into Fastlane, for example:

• http://docs.fastlane.tools/actions/setup_circle_ci
• http://docs.fastlane.tools/actions/setup_travis

There is a generic setup_ci action that seems to work, but there may be an opportunity to simplify the setup for folks by providing our own action.

http://docs.fastlane.tools/actions/setup_ci/