Added errors for understanding require problems in rabl and jbuilder

Author: gazay

lib/gon.rb

@@ -48,12 +48,18 @@ def clear
     end
 
     def rabl(*args)
+      unless Gon.constants.include?(:Rabl)
+        raise "Possible wrong require order problem - try to add `gem 'rabl'` before `gem 'gon'` in your Gemfile"
+      end
       data, options = Gon::Rabl.handler(args)
 
       store_builder_data 'rabl', data, options
     end
 
     def jbuilder(*args)
+      unless Gon.constants.include?(:Jbuilder)
+        raise "Possible wrong require order problem - try to add `gem 'jbuilder'` before `gem 'gon'` in your Gemfile"
+      end
       data, options = Gon::Jbuilder.handler(args)
 
       store_builder_data 'jbuilder', data, options