fix travis CI configuration for Xenial build
fix travis CI configuration for Xenial build

Adding:

    sh -e /etc/init.d/xvfb start

results in the following error:

    sh: 0: Can't open /etc/init.d/xvfb

see: https://benlimmer.com/2019/01/14/travis-ci-xvfb/
jprichardson/electron-mocha,jprichardson/electron-mocha