.vimrc: Avoid creating backups when editing files in `/tmp`

See http://vim.wikia.com/wiki/Editing_crontab.
.aliases: Remove the now-broken `whois` alias

Closes #397.
Merge branch 'master' of https://github.com/mathiasbynens/dotfiles

* 'master' of https://github.com/mathiasbynens/dotfiles:
  .aliases: Remove the now-broken `whois` alias
  .vimrc: Avoid creating backups when editing files in `/tmp`