Fix thread id collision detection not working in Python 2
Fix thread id collision detection not working in Python 2

hashlib returns a byte string in Python 2, but since we only use unicode
strings elsewhere the check against that in
SlackChannelHashedMessages.__missing__ didn't work. Make sure sha1_hex
returns a unicode string instead.

Fixes #800, closes #801

rawdigits/wee-slack,wee-slack/wee-slack