[git] Update `.gitignore` content

Remove the `Django` and `Sass` ignores, and update the `Vim` ones.
[git] Add comment about `color.ui` config variable

Starting with Git v1.8.4 the configuration variable `color.ui` defaults
to `auto`: http://git-blame.blogspot.no/2013/08/git-184.html.

Thanks @sindresorhus.