Fix: 'IRB is not a module (TypeError)'

Fixes #155 - IRB is not a module (TypeError) when starting zeus console.

Allows for use of Pry console in place of IRB. I've built and tested this and it works for me [and others](https://github.com/burke/zeus/issues/155#issuecomment-18617457).

All credit to @haarts for [this solution](https://github.com/burke/zeus/issues/155#issuecomment-16430481)!