Set up Mattermost webhook for Travis

[colindean]

• [X] @Justin417 needs to activate webhooks
• [X] Figure out webhook body
• [x] Put webhook into Travis build

The call should probably go in the after_script section of .travis.yml and look something like this:

curl -i -X POST -H 'Content-Type: application/json' \
  -d "$(jo channel=general-internal-chat username=travis-ci icon_url=https://travis-ci.com/images/logos/TravisCI-Mascot-1.png text="## This is a test\nThis is only a test")" \
  https://mm.example.org/hooks/XXXXXXXX

This is also worth considering: https://github.com/chatnhook/chatnhook

[colindean]

This will also require getting jo into Travis.

[colindean]

Theoretically b4ac541 should close this

[colindean]

This is done and working!