fix unhashable type caused by SmsApiException

[martinemplo]

If you override custom __eq__ method, you need to provide __hash__ as well. If not provided, raven crashes when handling the exception with: TypeError: unhashable type: 'SmsApiException' Minimal reproducible example:

>>> from smsapi.exception import SmsApiException
>>> try:
...     raise SmsApiException("emplocity")
... except SmsApiException as e:
...     print(hash(e))
... 

[in-void]

Cool, I'll keep that in mind. Thanks.