Cache node_modules in Travis CI

Closes js-cookiegh-444.

Author: DanielRuf

.travis.yml

@@ -1,6 +1,9 @@
 language: node_js
 node_js:
   - '6'
+cache:
+  directories:
+    - node_modules
 # Only use grunt-ci for commits pushed to this repo. Fall back to regular test
 # for pull requests (as secure variables won't be exposed there).
 script: