Stop testing with Node.js 6, as it's EOL

Author: marco-c

.travis.yml

@@ -15,7 +15,6 @@ env:
     - DISPLAY=$(if [[ $TRAVIS_OS_NAME == "linux" ]]; then echo ':99.0'; fi)
     - MOZ_XVFB=$(if [[ $TRAVIS_OS_NAME == "linux" ]]; then echo 1; fi)
   matrix:
-    - NODE_VERSION="6"  # Maintenance LTS
     - NODE_VERSION="8"  # Maintenance LTS
     - NODE_VERSION="lts/*"
     - NODE_VERSION="node"