Meta: Use Travis' container-based infrastructure

Add `sudo: false` to the `.travis.yml` file in order to use
Travis CI's container-based infrastructure, thus, making the builds
start and run faster.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

It seems this repository was recognize by Travis CI before 2015-01-01,
so builds where sent to the standard infrastructure:

 * https://docs.travis-ci.com/user/migrating-from-legacy/
 * https://docs.travis-ci.com/user/workers/container-based-infrastructure/

.travis.yml

@@ -9,3 +9,5 @@ language: node_js
 
 node_js:
   - "5"
+
+sudo: false