Remove xcpretty-travis-formatter dependency from Rakefile

See discussion [here](https://github.com/Automattic/simplenote-macos/pull/1007#discussion_r679634887).

This could be a good occasion to streamline the `Rakefile`, or even get rid of it in the first place and move all the tasks that we still use to `Fastlane`.

Start with the Rakefile and review the linked discussion in pull request 1007 to identify how xcpretty-travis-formatter is used. Check the existing Fastlane tasks before deciding whether only the dependency should be removed or whether the remaining Rake tasks should move there; done means the obsolete dependency is gone and the current tasks still work.

Written by the indexing model from the issue text.