Allow specifying package version for ruby-stomp

Add a new parameter $mcollective::ruby_stomp_ensure which defaults to
'installed'. Since this module manages the ruby-stomp package resource
it should also be possible to specify which package version to ensure.

Contains updated documentation and simple spec tests. Fixes MODULES-459.
Merge pull request #121 from antaflos/specify_ruby_stomp_version

Allow specifying pacakge version for ruby-stomp