MB-7250:  use "2.1.1" branch for 2.5.0.

This change does not change revision included, as HEAD of 2.1.1
branch is currently at 97fd781edaa109bf99f6f002ae5c562c8b829438.

Subsequent commits may advance the revision used.

Change-Id: I52c51e01f513efafa47983ea4a7f40e04e1f95d1
Reviewed-on: http://review.couchbase.org/30422
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>