Merge pull request #3739 from rionda/fix-completeopt-popup

Fix name of popup function

autoload/youcompleteme.vim

@@ -505,7 +505,8 @@ function! s:SetUpCompleteopt()
   " Also, having this option set breaks the plugin.
   set completeopt-=longest
 
-  if g:ycm_add_preview_to_completeopt ==# 'popup' && exists( '*popup_open' )
+  if g:ycm_add_preview_to_completeopt ==# 'popup' && 
+        \ ( v:version > 801 || ( v:version == 801 && has( 'patch1880' ) ) )
     set completeopt+=popup
   elseif g:ycm_add_preview_to_completeopt
     set completeopt+=preview