Fix for issue with Ruby 1.8.7 compatibility

Fixes the exception that you get when using Ruby 1.8.7, rabl, and gon.

In Ruby 1.8.7, Module.constants returned an array of strings (http://ruby-doc.org/core-1.8.7/Module.html#method-c-constants)

It looks like that changed to an array of symbols in Ruby 1.9.3+ (http://ruby-doc.org/core-1.9.3/Module.html#method-c-constants)

The code that checks to see if rabl or jbuilder is loaded depends on getting an array of symbols back. This causes the gem to fail on Ruby 1.8.7.