Move the Slack auth token from .travis.yml to the repository settings

[oliverklee]

According to https://docs.travis-ci.com/user/environment-variables/#Defining-Variables-in-Repository-Settings, secret environment variables that are the same across all branches and builds should be set in the repository settings, not in the .travis.yml.

@michield Would you be willing to take care of this for all four phpList 4 repositories?

[michield]

I think that's done for core now. But I'll check with the next few alerts to see if it was done correctly.