Rails should not be listed as dependency

[pashagray]

Gem has "Rails" dependency which is not correct. It should depend on something less general (e.g. Rack).

Because of this, we are not able to remove not used rails gems (e.g. actionmailbox). As soon as you set rails as dependency, all these gems are also included:

https://github.com/rails/rails/blob/de53ba56cab69fb9707785a397a59ac4aaee9d6f/rails.gemspec#L32