Skip default header for very old versions

Closes #386

Author: mhinz

autoload/startify.vim

@@ -64,7 +64,7 @@ function! startify#insane_in_the_membrane(on_vimenter) abort
  " Must be global so that it can be read by syntax/startify.vim.
  let g:startify_header = exists('g:startify_custom_header')
  \ ? s:set_custom_section(g:startify_custom_header)
- \ : startify#fortune#cowsay()
+ \ : (exists('*strwidth') ? startify#fortune#cowsay() : [])
  if !empty(g:startify_header)
  let g:startify_header += [''] " add blank line
  endif