Updated the Travis environment to xenial (#10090)
Updated the Travis environment to xenial (#10090)

* Updated the Travis environment to xenial.
* Python 2.6 needs to remain on Trusty (can't be installed from Xenial)

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