Update client.py

Group contributions are giving 404 errors with the appended `.json` extension, hence this crude hack.

Once this issue is fixed, we can remove this hack: https://github.com/soundcloud/soundcloud-python/issues/21

soundcloud/client.py

@@ -142,6 +142,8 @@ def _resolve_resource_name(self, name):
                 return '%s.json' % (name,)
             return name
         name = name.rstrip('/').lstrip('/')
+        if name[-13:] == 'contributions':
+            return '%s%s/%s' % (self.scheme, self.host, name)
         return '%s%s/%s.json' % (self.scheme, self.host, name)
 
     def _redirect_uri(self):