Update Vim 8 requirement

https://github.com/junegunn/vim-plug/issues/498#issuecomment-254239466

plug.vim

@@ -98,7 +98,7 @@ let s:plug_buf = get(s:, 'plug_buf', -1)
 let s:mac_gui = has('gui_macvim') && has('gui_running')
 let s:is_win = has('win32') || has('win64')
 let s:nvim = has('nvim') && exists('*jobwait') && !s:is_win
-let s:vim8 = has('patch-8.0.0001') && exists('*job_start')
+let s:vim8 = has('patch-8.0.0039') && exists('*job_start')
 let s:me = resolve(expand('<sfile>:p'))
 let s:base_spec = { 'branch': 'master', 'frozen': 0 }
 let s:TYPE = {