Adds back the python36 test

The python36 test was removed because the Docker file wasn't updating. This has now been resolved on Dockerhub: https://hub.docker.com/repository/docker/bilbydev/v2-dockerfile-test-suite-python36 so this adds back the test.

During the MR, the CPNest sampler was failing only the python 3.6 test due to this issue: https://github.com/johnveitch/cpnest/issues/62 to remedy this, I added resume=False. This isn't ideal as it doesn't test the full behaviour, but it is better to test some functionality than none.