Version 2.5 and above use the 2.1 version of tcmalloc

Change-Id: I5880bb13460a1687e42da8624dd1abb19a7d4425
Reviewed-on: http://review.couchbase.org/30649
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>