Fix Node.js version selection for Travis CI

See: https://docs.travis-ci.com/user/languages/javascript-with-nodejs

.travis.yml

@@ -1,4 +1,6 @@
 language: node_js
+
+node_js:
   - "node"
 
 dist: trusty
@@ -18,4 +20,4 @@ script:
 
 cache:
   directories:
-    - node_modules
+    - node_modules