Update YesNoField coercion

I am not sure if this is related to a py2/py3 change, or if it's
because django 1.11 changed the way it handles booleans; but either
way, this works.

pinaxcon/registrasion/forms.py

@@ -9,7 +9,7 @@ class YesNoField(forms.TypedChoiceField):
         kwargs['required'] = True
         super(YesNoField, self).__init__(
             *args,
-            coerce=lambda x: x is True,
+            coerce=lambda x: x in ['True', 'Yes', True],
             choices=((None, '--------'), (False, 'No'), (True, 'Yes')),
             **kwargs
         )