Build: Adjust nodejs versions in travis config

Remove version 9 and add version 12, so that all LTS version
will be checked.

Author: markusguenther

.travis.yml

@@ -7,8 +7,8 @@ cache:
 notifications:
   email: false
 node_js:
+  - '12'
   - '10'
-  - '9'
   - '8'
 before_install:
   - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.9.4