Bump required Ruby version to 2.1.0

[This article](http://weblog.rubyonrails.org/2014/8/20/Rails-4-2-beta1/#maintenance-consequences-and-rails-5-0) states that:

> Rails 5.0 is in most likelihood going to target Ruby 2.2.

Before the exact minimum version is fully decided, @arthurnn [suggests](#17830 (comment))
that **at least** version 2.1.0 **must** be required by the `gemspec` files.
If it's a hard requirement, should say "requires Ruby 2.1 or newer" rather than "prefers".