Speculative, for discussion: Add required Ruby version

While cleaning up environment variables in Jenkins I noticed that the vCloud Tools job injects the following environment variable: `RBENV_VERSION=1.9.3-p392`.

I've also noticed previously that our gems claim to work with any version of Ruby:

Ruby Dependency

    >= 0

http://rubygems.org/gems/vcloud-tools.

I'm guessing this is not the case! So we should set that. Is it `1.9.3`? Does anyone want to test this or can we assume it? Should we add this to all the gems? Discuss.