botogram 0.2.x crashes with edited messages

An user reported this crash on the Telegram group:

```
Traceback (most recent call last):
  File "C:\...\botogram\runner\processes.py", line 54, in run
    self.loop()
  File "C:\...\botogram\runner\processes.py", line 207, in loop
    if update.message.date < self.started_at:
AttributeError: 'NoneType' object has no attribute 'date'
```

**botogram version**: 0.2.1
