Nvim: do not set 'shortmess' for Nvim v0.3.4

Nvim v0.3.4 bug: https://github.com/neovim/neovim/issues/9885

Since the `shortmess+=I` is just a workaround for a potentional flicker at start
itself, it now requires either Vim or at least Nvim v0.3.5.

A fix was later backported to v0.3.4, but we can't rely on that.

References https://github.com/mhinz/vim-startify/issues/369

plugin/startify.vim

@@ -10,7 +10,8 @@ endif
 let g:loaded_startify = 1
 let g:startify_locked = 0
 
-if !get(g:, 'startify_disable_at_vimenter')
+if !get(g:, 'startify_disable_at_vimenter') && (!has('nvim') || has('nvim-0.3.5'))
+  " Only for Nvim v0.3.5+: https://github.com/neovim/neovim/issues/9885
   set shortmess+=I
 endif