Tell Grunt/jshint to ignore `src/bp-forums/bbpress/`.

This folder is only present on SVN checkouts, but for people trying to contribute to BP using SVN, jshint was failing when it was trying to parse the old bundled version of bbPress.

See #6015


git-svn-id: https://buddypress.svn.wordpress.org/trunk@9146 cdf35c40-ae34-48e0-9cc9-0c9da1808c22