Nvim: adjust VimEnter check due to upstream change

Due to a recent change in Nvim [1], our VimEnter check wasn't firing anymore and
thus Startify wouldn't run at startup.

The line2byte() "trick" was replaced by less abusing VimL.

[1]: neovim/neovim@b051b13

Fixes #549

Author: mhinz

plugin/startify.vim

@@ -34,7 +34,7 @@ function! s:update_oldfiles(file)
 endfunction
 
 function! s:on_vimenter()
-  if !argc() && line2byte('$') == -1
+  if !argc() && line('$') == 1 && getline('.') == ''
     if get(g:, 'startify_session_autoload') && filereadable('Session.vim')
       source Session.vim
     elseif !get(g:, 'startify_disable_at_vimenter')