Switch couchdb from commit to branch

Instead of using a certain commit, we are now using the correct
branch for couchdb.

Change-Id: I494969862f4f8fb081f77e133a8e80c8f6403706
Reviewed-on: http://review.couchbase.org/37546
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>