Use better syntax to compare gem version

Author: igorkasyanchuk

lib/devise/failure_app.rb

@@ -279,7 +279,7 @@ def root_path_defined?(context)
     end
 
     def rails_51_and_up?
-      Rails::VERSION::MAJOR >= 6 || (Rails::VERSION::MAJOR >= 5 && Rails::VERSION::MINOR >= 1)
+      Rails.gem_version >= Gem::Version.new("5.1")
     end
   end
 end