Mattermostdriver

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Update requirements:
- requests2.20.0

'''''
Added Endpoints:
- Reactions

Thanks a lot to aedho3yn for this!

'''''
Fixes:
- Unable to create custom emojies 33
- Use ResourceNotFound for 404 34

Thanks a lot to Sakarah and maxbrunet for these fixes!

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POSSIBLE BREAKING CHANGES:
Requirements have been updated:
- websockets==6.0

This requirement (and requests from 5.0.0) are now included in the setup.py file.
If you didn't need to update before it is possible you do now,
which could cause a breaking change this time.
Not exactly sure about this, but better safe then sorry!

'''''
Fixes:
- Do not try to parse responses as json when it's clear they are not. Thanks Kenan!

'''''
POSSIBLE BREAKING CHANGES:
Requirements have been updated:
- requests==2.19.1
- websockets==5.0

Fixes:
- The error message 'Websocket authentication failed' was not always correct