Avoid API Limit from composer downloads on github

Currently quite some travis tests fail with:

Could not fetch https://api.github.com/repos/sebastianbergmann/phpunit-mock-objects/zipball/1.2.3, enter your GitHub credentials to go over the API rate limit

The credentials will be swapped for an OAuth token stored in /home/travis/.composer/config.json, your password will not be stored

To revoke access to this token you can visit https://github.com/settings/applications

Username:

No output has been received in the last 10 minutes, this potentially indicates a stalled build or something wrong with the build itself.

The build has been terminated

https://github.com/composer/composer/issues/1314 states some options to circumvent that.