check for global constant instead of accidentily autoloading

when checking for `AwesomePrint` in the Cistern scope, it was trying to autoload.

was causing:

```
LoadError: cannot load such file -- awesome_print
	from /opt/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /opt/chef/embedded/lib/ruby/site_ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/cistern-0.3.1/lib/cistern/formatter/awesome_print.rb:1:in `<top (required)>'
	from /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/cistern-0.3.1/lib/cistern/formatter.rb:8:in `default'
	from /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/cistern-0.3.1/lib/cistern.rb:25:in `formatter'
	from /opt/chef/embedded/lib/ruby/gems/1.9.1/gems/cistern-0.3.1/lib/cistern/collection.rb:52:in `inspect'
```