return boolean for is_mobile_request?

I'm using Rails 4 with Ruby 2.0.0, and the helper method `is_mobile_request?` is returning the result of `request.user_agent.to_s.downcase =~ @@mobylette_options[:mobile_user_agents].call`. When the user agent doesn't match, the method returns `nil`. When the user agent is mobile, it returns the match in `:mobile_user_agents` (for iphone, this is 13).

Because of this, mobylette was not correctly rendering mobile templates. After this change, everything is working well.