Use "git describe" as VERSION

Change-Id: Ic4d7ed4e9fbbdf669cb6fec0a2864ef44d98635a
Reviewed-on: http://review.couchbase.org/30655
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>