Change gemspec to not depend on the whole of rails

[mikebaldry]

Many apps only pull in the necessary rails gems for what they need, for instance not using actionmailbox, which has a whole load of other dependencies.

interactor-rails depends on rails unnecessarily. I think it probably only needs to depend on railties?

I've had to stop using interactor-rails because it forces my app to depend on tons of extra libraries that aren't used.