Add node_modules cache to travis

This could speed up CI builds a little bit

See https://docs.travis-ci.com/user/caching/ for more information.

Author: weswigham

.travis.yml

@@ -6,3 +6,7 @@ node_js:
   - '0.10'
 
 sudo: false
+
+cache:
+  directories:
+    - node_modules