CBLT-255:  change 'last_done_and_ios_bld' to 'last_done_mobile_bld'

Change-Id: I86cffc18a054e2442721cb88b3ac5ac77a44d2b2
Reviewed-on: http://review.couchbase.org/40034
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>