Use `gui_macvim` instead of `mac` (#36)

plug.vim

@@ -58,7 +58,7 @@ set cpo&vim
 let s:plug_source = 'https://raw.github.com/junegunn/vim-plug/master/plug.vim'
 let s:plug_file = 'Plugfile'
 let s:plug_buf = -1
-let s:mac_gui = has('mac') && has('gui_running')
+let s:mac_gui = has('gui_macvim') && has('gui_running')
 let s:is_win = has('win32') || has('win64')
 let s:me = expand('<sfile>:p')