devel/rubygem-gettext_i18n_rails_js: Add rubygem-gettext_i18n_rails_js 2.1.0

GettextI18nRailsJs extends gettext_i18n_rails, making your .PO files available
to client side Javascript as JSON. It will find translations inside your .js,
.coffee, .handlebars and .mustache files, then it will create JSON versions of
your .PO files so you can serve them with the rest of your assets, thus letting
you access all your translations offline from client side Javascript.