rails: detect gems.rb in _rake_command (#8223)

plugins/rails/rails.plugin.zsh

@@ -17,7 +17,7 @@ function _rake_command () {
     bin/stubs/rake $@
   elif [ -e "bin/rake" ]; then
     bin/rake $@
-  elif type bundle &> /dev/null && [ -e "Gemfile" ]; then
+  elif type bundle &> /dev/null && ([ -e "Gemfile" ] || [ -e "gems.rb" ]); then
     bundle exec rake $@
   else
     command rake $@