Re-introduce the prometheus version update to 1.6.x
This had been originally added in https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/1495
And reverted in 077c6a76 due to dependancy on go 1.8
We have since upgrade our builds to go 1.8
Closes: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2243
cc\ @marin @bjk-gitlab