Add Gentoo support

Introduces dependency to gentoo/portage module Package installation happens
with portage::package, in order to handle KEYWORDS and USE flags for the
Gentoo packages for dev-lang/ruby and dev-ruby/rubygems

Signed-off-by: Theo Chatzimichos <tampakrap@gentoo.org> Signed-off-by:
Vikraman Choudhury <vikraman@gentoo.org>