Changed Travis CI environment to cache node_modules folder

This is a commonly used trick to speed up Travis CI builds. Since
builds for this project tend to spend upwards of 40 seconds in the
`npm install` phase, this has the potential to be a big time saver.

Author: stevenbenner

.travis.yml

@@ -1,6 +1,9 @@
 language: node_js
 node_js:
   - "node"
+cache:
+  directories:
+    - node_modules
 notifications:
   irc:
     channels: