Cache node_modules on Travis

This isn't actually enabled yet for public projects, but this will help
us speed up builds when it does get enabled.

http://docs.travis-ci.com/user/caching/

.travis.yml

@@ -2,6 +2,9 @@
 language: node_js
 node_js:
 - '0.10'
+cache:
+  directories:
+  - node_modules
 script:
 - |
     grunt $TEST_TYPE