Fix inbound.attachments error when no attachments available

Traceback:
```
File "/home/eka/python/experiments/local/lib/python2.7/site-packages/mandrill_inbound/__init__.py", line 89, in attachments
    for name, attachment in self.msg.get('attachments').items():
AttributeError: 'NoneType' object has no attribute 'items'
```