cbdeps: Only consider CentOS major version

Change-Id: I9b74a8720f23f4259a31099e993928a9f741d6bf
Reviewed-on: http://review.couchbase.org/44421
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: David Haikney <david.haikney@couchbase.com>