Attempt to update the commit status for GitLab builds

When GitLab is used to as the backing scm provider try to call the [API](http://docs.gitlab.com/ce/api/commits.html#commit-status) to update the status of the commit. Handle 403/404 failures gracefully and continue running even if the call fails. This will allow users who are in a position to run danger with developer permissions to do so and gracefully degrade for users that run danger with guest or reporter permissions.

cc @orta