Update WebhookChannel to use the json request option in Guzzle

[kylestev]

The json option essentially does what the code is doing now (calling json_encode) along with setting the Content-Type header to application/json.

---

This will automatically set the appropriate Content-Type header[1]

[1] http://docs.guzzlephp.org/en/stable/request-options.html#json

[kylestev]

Style CI does not pass but I did resolve the issues in #11 if that is merged.

[atymic]

ping @kylestev