ci: run build on node.js >= 10

Dropping support for nodejs<=8 in CI, since `package.json` specifies to install the latest karma, and it doesn't support old nodejs https://github.com/karma-runner/karma/blob/master/CHANGELOG.md#500-2020-04-09

.travis.yml

@@ -4,13 +4,9 @@ cache:
   directories:
     - node_modules
 node_js:
-  - '4'
-  - '5'
-  - '6'
-  - '7'
-  - '8'
+  - '10'
   - 'stable'
+services:
+  - xvfb
 before_script:
-  - export DISPLAY=:99.0
-  - sh -e /etc/init.d/xvfb start
   - npm install -g grunt-cli