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>