In older vim versions, noinsert and noselect didn't work

Use longest instead of them.

autoload/jedi.vim

@@ -569,11 +569,18 @@ function! jedi#complete_string(autocomplete) abort
         set completeopt+=menuone
         set completeopt-=menu
         if &completeopt !~# 'noinsert\|noselect'
+            " Patch 775 introduced noinsert and noselect, previously these
+            " options didn't exist. Setting them in earlier versions results in
+            " errors (E474).
+            if has("patch-7.4-775")
                 if g:jedi#popup_select_first
                     set completeopt+=noinsert
                 else
                     set completeopt+=noselect
                 endif
+            else
+                set completeopt+=longest
+            endif
         endif
     elseif pumvisible()
         return "\<C-n>"