This method is only available on newer versions.
The first available version (v2_6_3) is shown here.
install_rubygems(names, version, sources)
private
Installs the plugin from rubygems source and returns the path where the
plugin was installed
@param [String] name of the plugin gem to search
in the source @param [Array] version of the gem
to install @param [String,Array]
source(s) to resolve the gem
@return [Hash] map of names to the specs of
plugins installed
