Switch back to main-ruby-go docker image for tests
Switch back to main-ruby-go docker image for tests

[#150807313](https://www.pivotaltracker.com/story/show/150807313)

Signed-off-by: Dale Wick <188d987a4b971a1fecf708bf002a3b94c86c96a2@pivotal.io>

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