Add plugin spec

Whilst I've been trying to get more test coverage of some of my plugins, I found some nice ways of testing some common vagant plugin stuff. Just thought I should add it to vagrant-omnibus! :+1: 
Is it really needed to lock these down to minor (instead of major) versions? Especially Rake version shouldn't matter that much, should it?
With rubocop I understand the need, been there. =)