Add support for rich_text field

[timfjord]

Currently, there is no way to define a rich_text field via migration.
This PR adds support for that

class CreateFooContentType < ActiveRecord::Migration
  include ContentfulModel::Migrations::Migration

  def up
    create_content_type('foo') do |ct|
      ct.field('bar', :rich_text)
    end
  end

  def down
  end
end

[dlitvakb]

Thanks for the contribution! Merging!