ci: use newer node

mgechev · mgechev · commit f571b1a5d3b8 · 2017-06-07T14:18:33.000-04:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - "4"
+  - "8"
 before_script:
   - npm install -g gulp
 script: gulp build
