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.

test/sampler_test.py

@@ -526,7 +526,7 @@ class TestRunningSamplers(unittest.TestCase):
     def test_run_cpnest(self):
         _ = bilby.run_sampler(
             likelihood=self.likelihood, priors=self.priors, sampler='cpnest',
-            nlive=100, save=False)
+            nlive=100, save=False, resume=False)
 
     def test_run_dynesty(self):
         _ = bilby.run_sampler(