Grunt: rename task `grunt build-dev` to `grunt build`.

This is the default task that runs when you run just `grunt` so it doesn't break any existing workflows for us,
but makes the development Grunt task name consistent with bbPress and WordPress.

Props netweb, see #5160

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8390 cdf35c40-ae34-48e0-9cc9-0c9da1808c22