fix: allow cycles in module dependencies

Although hardly a good practice it might happen that we can have
cycles in module dependencies. Node.js is handling this situation
as described here: http://nodejs.org/api/modules.html#modules_cycles
The simple fix from this commit allows similar behavior.

On the project I'm playing with I've bumped into such situation that
was handled correctly by a module loader we are using but failed 
with stack overflow while using this plugin.