Do not check Ruby interface when parallel update is not needed

In case Ruby interface is completely broken that simply running
`:ruby require 'thread'` crashes Vim.

Close #564

plug.vim

@@ -955,7 +955,7 @@ function! s:update_impl(pull, force, args) abort
 
   let use_job = s:nvim || s:vim8
   let python = (has('python') || has('python3')) && !use_job
-  let ruby = has('ruby') && !use_job && (v:version >= 703 || v:version == 702 && has('patch374')) && !(s:is_win && has('gui_running')) && s:check_ruby()
+  let ruby = has('ruby') && !use_job && (v:version >= 703 || v:version == 702 && has('patch374')) && !(s:is_win && has('gui_running')) && threads > 1 && s:check_ruby()
 
   let s:update = {
     \ 'start':   reltime(),