Use system() instead of systemlist() for older versions of Vim

Fix #1184

autoload/fzf/vim.vim

@@ -72,7 +72,7 @@ function! s:check_requirements()
     throw "fzf#exec function not found. You need to upgrade Vim plugin from the main fzf repository ('junegunn/fzf')"
   endif
   let exec = fzf#exec()
-  let output = systemlist(exec . ' --version')
+  let output = split(system(exec . ' --version'), "\n")
   if v:shell_error || empty(output)
     throw 'Failed to run "fzf --version": ' . string(output)
   endif