Updating centos-6-builddocker to be based on recommended release: CentOS 6.3

Change-Id: Ic79742d6b9042e247daa640749690058d1de15b7
Reviewed-on: http://review.couchbase.org/40717
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>