Vim: explicitly check for job support.

autoload/gitgutter/async.vim

@@ -10,10 +10,12 @@ let s:jobs = {}
 " MacVim-GUI didn't support async until 7.4.1832 (actually commit
 " 88f4fe0 but 7.4.1832 was the first subsequent patch release).
 let s:available = has('nvim') || (
+      \ has('job') && (
 		  \ (has('patch-7-4-1826') && !has('gui_running')) ||
 		  \ (has('patch-7-4-1850') &&  has('gui_running')) ||
 		  \ (has('patch-7-4-1832') &&  has('gui_macvim'))
 		  \ )
+      \ )
 
 function! gitgutter#async#available()
   return s:available