Update stemcells/centos_spec test to 6.6
Update stemcells/centos_spec test to 6.6

Signed-off-by: Zachary Auerbach <d52987198ea2730fd22a38e7976344d843a7ffa0@zaksoup.com>

barthy1/bosh,barthy1/bosh,barthy1/bosh,barthy1/bosh