Grunt: sync `.gitignore` against SVN's svn:ignore property.

Now, both are finally in sync. :)

This commit is part of a series that will re-organise BuddyPress trunk to use a Grunt build process. See #5160


git-svn-id: https://buddypress.svn.wordpress.org/trunk@8360 cdf35c40-ae34-48e0-9cc9-0c9da1808c22