Enable Travis CI's new container-based infrastructure

This is being done in order to enable Travis's new caching functionality
and thus to speed up the build. See previous commit adding the `cache`
section to .travis.yml, and see also:

http://docs.travis-ci.com/user/workers/container-based-infrastructure/

.travis.yml

@@ -1,3 +1,5 @@
+sudo: false
+
 language: java
 jdk: oraclejdk8