Update node version.

.travis.yml

@@ -1,14 +1,16 @@
 language: node_js
+
 sudo: false
+
 node_js:
-- '6'
+  - '8'
-- '7'
+
 before_install:
-  - npm install -g bower
   - export DISPLAY=:99.0
   - sh -e /etc/init.d/xvfb start
+
 install:
-  - bower install
   - npm install
+
 after_script:
-  - gulp coveralls
+  - npm coveralls