Fixed GoCD dockerfile for unit & integration tests

To run:
- cd bosh
- RUBY_VERSION=2.1.2 gocd/bosh/run-in-container.sh /opt/bosh/tests/unit/job.sh
- RUBY_VERSION=2.1.2 gocd/bosh/run-in-container.sh /opt/bosh/tests/integration/job.sh

[#76642222]