Remove a reference to Ruby 1.9 in Guides

[ci skip]

Now that Rails requires Ruby >= 2.0 there is no need to document
Ruby 1.9.
Merge pull request #17838 from claudiob/remove-ruby19-reference-in-guides

Remove a reference to Ruby 1.9 in Guides
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](https://github.com/rails/rails/pull/17830#issuecomment-64940383)
that **at least** version 2.1.0 **must** be required by the `gemspec` files.