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

Fixes #155 - IRB is not a module (TypeError) when starting zeus console.

All credit to @haarts for this!
https://github.com/burke/zeus/issues/155#issuecomment-16430481
Ruby environment setup

Specify ruby 1.9.3-p429 as target version for rbenv and avoid error
'Your Gemfile has no gem server sources.' during make.
Merge pull request #455 from skillstream/master

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