Fix Cert_URI Bug to string for Hangouts Channel

[Younesegh]

Proposed changes:

• Changed the CERT_URI in hangouts.py to a string.

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• [Fixed this error traceback (most recent call last):

  File "/home/ec2-user/.local/lib/python3.6/site-packages/sanic/app.py", line 976, in handle_request
    response = await response
  File "/home/ec2-user/.local/lib/python3.6/site-packages/rasa/core/channels/hangouts.py", line 280, in receive
    self._check_token(token)
  File "/home/ec2-user/.local/lib/python3.6/site-packages/rasa/core/channels/hangouts.py", line 257, in _check_token
    bot_token, self.project_id, cert_uri=CERT_URI,
  File "/home/ec2-user/.local/lib/python3.6/site-packages/oauth2client/_helpers.py", line 133, in positional_wrapper
    return wrapped(*args, **kwargs)
  File "/home/ec2-user/.local/lib/python3.6/site-packages/oauth2client/client.py", line 1556, in verify_id_token
    resp, content = transport.request(http, cert_uri)
  File "/home/ec2-user/.local/lib/python3.6/site-packages/oauth2client/transport.py", line 282, in request
    connection_type=connection_type)
  File "/home/ec2-user/.local/lib/python3.6/site-packages/httplib2/__init__.py", line 1794, in request
    (scheme, authority, request_uri, defrag_uri) = urlnorm(uri)
  File "/home/ec2-user/.local/lib/python3.6/site-packages/httplib2/__init__.py", line 241, in urlnorm
    (scheme, authority, path, query, fragment) = parse_uri(uri)
  File "/home/ec2-user/.local/lib/python3.6/site-packages/httplib2/__init__.py", line 236, in parse_uri
    groups = URI.match(uri).groups()
TypeError: expected string or bytes-like object ] 

• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[CLAassistant]

All committers have signed the CLA.

[sara-tagger]

Thanks for submitting a pull request 🚀 @tmbo will take a look at it as soon as possible ✨

[tmbo]

Thanks a lot for the fix - looks good.

Do you mind creating a changelog entry? once that is done we are ready to merge.