Travis: Enable IPv6 support for Docker
    
    Environment variable 'TRAVIS' is not accessible inside the container
    anymore, passing it to the container didn't have the same effect
    expected by the hack this commit removes. However, enabling IPv6, as
    described in https://github.com/travis-ci/travis-ci/issues/8891,
    was effective to execute the tests.
    
    Issue: https://pagure.io/freeipa/issue/8213
    
    Signed-off-by: Armando Neto <abiagion@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>