has('win32') is enough for detecting Windows (#765)

plug.vim

@@ -96,7 +96,7 @@ let s:plug_src = 'https://github.com/junegunn/vim-plug.git'
 let s:plug_tab = get(s:, 'plug_tab', -1)
 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:is_win = has('win32')
 let s:nvim = has('nvim-0.2') || (has('nvim') && exists('*jobwait') && !s:is_win)
 let s:vim8 = has('patch-8.0.0039') && exists('*job_start')
 let s:me = resolve(expand('<sfile>:p'))