Change Gitter room of Travis build notifications

[ceedubs]

I noticed that even thought there is a cats-mtl Gitter channel, the Travis build notifications are going to the Cats Gitter channel instead. I don't have write permissions to this repo, so I can't see the right URL to fix it, but someone with permissions should be able to go to the cats-mtl Gitter channel, click the settings knobs on the upper right, click Integrations and set up a new webhook URL for Travis.

If you leave the URL hard-coded in the .travis.yml then you will still get notifications for builds on forks. Also I don't think that there's anything preventing anyone from copying the URL and sending build notifications for unrelated projects (though I don't know what their motive would be). See typelevel/cats#2691 for how to easily transition to the preferred way of setting up these notifications.