amended the previously missed Bigip_Failover_State class to inherit from ResourceBase. Minor code change to rectify travis error

Author: wojtek0806

f5/bigip/resource.py

@@ -315,9 +315,10 @@ def _check_exclusive_parameters(self, **kwargs):
             ex_set = set(self._meta_data['exclusive_attributes'][0])
             common_set = attr_set.intersection(ex_set)
             if len(common_set) > 1:
+                cset = ', '.join(common_set)
                 error = 'Mutually exclusive arguments submitted. ' \
                         'The following arguments cannot be set ' \
-                        'together: "{}".'.format(', '.join(common_set))
+                        'together: "%s".' % cset
                 raise ExclusiveAttributesPresent(error)
 
     @staticmethod