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

mhinz/vim-startify