You can't md5 a unicode

Fixes https://app.getsentry.com/sentry/sentry/group/6511458/

```python
>>> hashlib.md5(u'ü').hexdigest()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeEncodeError: 'ascii' codec can't encode character u'\xfc' in position 0: ordinal not in range(128)
>>> hashlib.md5(u'ü'.encode('utf8')).hexdigest()
'c03410a5204b21cd8229ff754688d743'
```