ruby-ng.eclass: add missing 'estack' inherit to EAPI=6

ruby-ng.eclass: add missing 'estack' inherit to EAPI=6

Noticed when ran egencache:
 $ PORTAGE_ELOG_CLASSES="qa" egencache --repo=gentoo --update --update-use-local-desc --jobs=$(nproc)
 * Call stack:
 *        ruby-ng.eclass, line 142:  Called command_not_found_handle 'eshopts_push' '-o' 'noglob'
 *              ebuild.sh, line 325:  Called __qa_source '/gentoo-ebuilds/gentoo/eclass/ruby-fakegem.eclass'
 *             ebuild.sh, line 624:  Called source '/gentoo-ebuilds/gentoo/dev-ruby/moneta/moneta-1.1.0.ebuild'
 *             ebuild.sh, line  89:  Called die

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>