Build: Remove node version 6 from travis config

Author: Markus Günther

.travis.yml

@@ -10,7 +10,6 @@ node_js:
   - '10'
   - '9'
   - '8'
-  - '6'
 before_install:
   - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.9.4
   - export PATH="$HOME/.yarn/bin:$PATH"