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

@@ -79,14 +79,16 @@ class Emcee(MCMCSampler):
         function_kwargs['p0'] = self.pos0
 
         if LooseVersion(emcee.__version__) > LooseVersion('2.2.1'):
-            for key in updatekeys:
-                function_kwargs[updatekeys[key]] = function_kwargs.pop(key)
-
             if function_kwargs['mh_proposal'] is not None:
                 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)
+                else:
+                    del function_kwargs[key]
 
         return function_kwargs