use ruby-2.4.2 for tests
use ruby-2.4.2 for tests

[#150905630](https://www.pivotaltracker.com/story/show/150905630)

Signed-off-by: J Evelyn <d27bc0d2d15da5ab73e41c4bc24f60b09b05fd67@pivotal.io>

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