Upgrade to Electron 6:

New version of Electron has enable by default sandbox
http://www.atom.pe/docs/api/sandbox-option/

There was some issues to migrate a new version of Electron for
MagicMirror. Using the new version in Travis CI was failing at this
time. The problem is because the testing runner is a Docker enviroment

The issue experimented is the same topic mentioned here:
 - electron/electron#17972
 - electron-userland/spectron#443

The fix for to all of this is to set the `--no-sandbox` mode in CI
testing https://electronjs.org/docs/all#--no-sandbox

This change use the feature to set and disable Sandbox using
by enviroment variable `ELECTRON_DISABLE_SANDBOX=1`
electron/electron#16576

This change has reference MagicMirrorOrg#1800

.travis.yml

@@ -8,6 +8,7 @@ before_script:
  - yarn danger ci
  - npm install grunt-cli -g
  - "export DISPLAY=:99.0"
+ - "export ELECTRON_DISABLE_SANDBOX=1"
  - "sh -e /etc/init.d/xvfb start"
  - sleep 5
 script: