Rails 5 and Gemfile upgrade

[toastercup]

• Thanks to constant upgrading of our Gemfile, upgrading to Rails 5 is easy! All dependencies now support Rails 5.
• Grape 0.18 locks mustermann-grape to 0.4.0, which conflicts with sinatra 2, which depends on mustermann 0.4.0. Grape 0.17, for whatever reason, does not lock down this gem, and it all still works. They have a storied history of locking down gems after-the-fact, on newer versions of Grape.. See this PR for the lockdown: Drop ruby-2.0 and mustermann19, use mustermann-grape instead ruby-grape/grape#1470
• config.active_record.raise_in_transactional_callbacks is no longer relevant in Rails 5