CBD-1249:  add support for centos-6-x64

Change-Id: I8a4649083ff99fce22fad22e6d7d7f2229af3490
Reviewed-on: http://review.couchbase.org/39058
Reviewed-by: Philip Labee <philip@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>