CI: Switch to Container-Based Travis

This commit switches the Travis tests over to Travis's new
container-based infrastructure.  It also tells Travis to
cache the node_modules directory for faster setup.

.travis.yml

@@ -1,4 +1,8 @@
 language: node_js
+sudo: false
+cache:
+  directories:
+  - node_modules
 node_js:
 - '0.11.13'
 env: