gem plugin: added _files to install command

is useful if you want to install a local gem from the filesystem

plugins/gem/_gem

@@ -59,6 +59,8 @@ case "$words[1]" in
   build)
     _files -g "*.gemspec"
     ;;
+  install)
+    _files ;;
   list)
       if [[ "$state" == forms ]]; then
         _gem_installed