Set Vim's spellfile to $HOME

Running `zg` adds words to the `spellfile`:

https://github.com/thoughtbot/dotfiles/commit/4f5a2edc336970b8a1c7eded3934182810abeb20
http://robots.thoughtbot.com/vim-spell-checking

Setting the spellfile keeps it out of its default location, `vim/spell`, which
would otherwise be inside thoughtbot/dotfiles. We don't necessarily want to
share the `spellfile` across the team.