cache travis node modules

This will speed up builds by several minutes, but we will have to clear
the cache when dependencies are updated.
See https://docs.travis-ci.com/user/caching/#Clearing-Caches

.travis.yml

@@ -3,6 +3,9 @@ node_js:
   - "7"
 dist: trusty
 sudo: required
+cache:
+  directories:
+    - node_modules
 before_install:
   - export DISPLAY=:99.0
   - sh -e /etc/init.d/xvfb start