jenkins_plugin fixes/improvements

* Properly allow installation of specific versions of a plugin. Previously this only worked when a source URL was provided.
* Optionally ensure a plugin’s dependencies are installed before proceeding with it’s installation. This is enabled by default.
* Handle plugin downgrades correctly (requires an uninstall of existing, newer version).

Fixes #259, #260, #269, #271, #276

/cc @opscode-cookbooks/release-engineers @epu @fletchowns @dcheese @xbeta @cbek @pghalliday @spajus 
'dependencies' /nitpick