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

@@ -93,7 +93,7 @@ class Emcee(MCMCSampler):
     @property
     def sampler_init_kwargs(self):
         import emcee
-        
+
         init_kwargs = {key: value
                        for key, value in self.kwargs.items()
                        if key not in self.sampler_function_kwargs}
@@ -138,7 +138,7 @@ class Emcee(MCMCSampler):
     @property
     def nwalkers(self):
         return self.kwargs['nwalkers']
-   
+
     @property
     def nsteps(self):
         return self.kwargs['iterations']