dev-ruby/erubi: keyword ~arm, ~ppc, ~x86

This package is a simplified version of dev-ruby/erubis
which already has keywords for these arches. Adding them
now avoids a large keywording bug for dev-ruby/rails:5.1.

Package-Manager: Portage-2.3.5, Repoman-2.3.1

dev-ruby/erubi/erubi-1.6.0.ebuild

@@ -17,7 +17,7 @@ HOMEPAGE="https://github.com/jeremyevans/erubi"
 
 LICENSE="MIT"
 SLOT="0"
-KEYWORDS="~amd64"
+KEYWORDS="~amd64 ~arm ~ppc ~x86"
 IUSE=""
 
 ruby_add_bdepend "test? ( dev-ruby/minitest )"