Update plugin.pp

MODULES-453: When a package plugin is specified, we should be able to set the value of ensure through a variable called version.
Update plugin.pp
Merge pull request #120 from kedlav/patch-1

Allow plugin version control
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.