Improve check of Neovim Job API

Checking for the existence of `jobwait()` function seems like a more
robust way to check if the current Neovim process meets the requirement
of the parallel installer. This condition precludes a very old version
of Neovim built even before the initial Job API was implemented.

plug.vim

@@ -74,7 +74,7 @@ let s:mac_gui = has('gui_macvim') && has('gui_running')
 let s:is_win = has('win32') || has('win64')
 let s:py2 = has('python') && !has('nvim') && !s:is_win && !has('win32unix')
 let s:ruby = has('ruby') && !has('nvim') && (v:version >= 703 || v:version == 702 && has('patch374'))
-let s:nvim = has('nvim') && !exists('##JobActivity') && !s:is_win
+let s:nvim = has('nvim') && exists('*jobwait') && !s:is_win
 let s:me = resolve(expand('<sfile>:p'))
 let s:base_spec = { 'branch': 'master', 'frozen': 0 }
 let s:TYPE = {