cbdeps: Only consider CentOS major version

Change-Id: I5287106bedec3d0e460a7b7d5944d69a4e7e0d49
Reviewed-on: http://review.couchbase.org/44082
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Chris Hillery <ceej@couchbase.com>