Configure Zeus tokens in Travis automatically

[jan-auer]

Our Travis provider could automatically create secure environment variables so the users don't have to do that manually:

• ZEUS_HOOK_BASE: Currently used for artifact upload, might become unnecessary with Upload artifacts via API with token #50
• ZEUS_TOKEN: Repository token (zeus-r-<hash>) that only grants access to this repo

See https://docs.travis-ci.com/api/#settings:-environment-variables.

However, the repository must be enabled in travis at that time already. I've found no documented way to do this via the API, but the travis command line tool can do that.