Update VERSION_STRING in autoconf to 2.3.0-dp2

Change-Id: Iee573bdd88b0aacb24186c141cc7c546dd2082c1
Reviewed-on: http://review.couchbase.org/33999
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>