Update emcee.py: slight fix in case passing newer argument names as keywords

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