work around an issue with docker/dockerhub (#10547)
work around an issue with docker/dockerhub (#10547)

* try to workaround an issue with docker/dockerhub
* apply workaround only when running in travis

LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack,iulian787/spack