Allow emcee to work with pre-release versions

The release version of emcee (2.2.1) is over two years old, however there are a couple of more recent pre-releases available on PyPI. These releases change the names of a few arguments, so in this PR I've made some changes that should allow the use of these newer versions of emcee.

bilby/core/sampler/emcee.py

@@ -83,7 +83,7 @@ class Emcee(MCMCSampler):
                 logger.warning("The 'mh_proposal' option is no longer used "
                                "in emcee v{}, and will be ignored.".format(emcee.__version__))
             del function_kwargs['mh_proposal']
-            
+
             for key in updatekeys:
                 if updatekeys[key] not in function_kwargs:
                     function_kwargs[updatekeys[key]] = function_kwargs.pop(key)