Start using new Travis container-based infrastructure.

Author: BernardoSilva

.travis.yml

@@ -13,3 +13,7 @@ before_script:
 script:
   - vendor/bin/phpunit
   - vendor/bin/phpcs --standard=PSR2 ./src/
+
+# Use Travis' new container-based infrastructure.
+# See http://docs.travis-ci.com/user/migrating-from-legacy/#How-can-I-use-container-based-infrastructure%3F
+sudo: false