update omnibus to 5.6.1
update omnibus to 5.6.1

Updates to a version of omnibus that automates populating the S3 cache.
https://github.com/chef/omnibus/pull/781

Signed-off-by: Robb Kidd <11cd4f9ba93acb67080ee4dad8d03929f9e64bf2@chef.io>
chef/supermarket,robbkidd/supermarket,robbkidd/supermarket,chef/supermarket,chef/supermarket,chef/supermarket,chef/supermarket,robbkidd/supermarket,robbkidd/supermarket