chore(travis): set 'sudo: false' and reenable travis cache

For some reason our jobs are being routed to the 'build.linux' travis queue, which has the travis cache
disabled. In order for us to get the cache reenabled we need to get our jobs into the 'builds.docker' queue.
This can be achieved via setting 'sudo: false' in .travis.yaml

.travis.yml

@@ -1,4 +1,5 @@
 language: node_js
+sudo: false
 node_js:
   - '0.10'