[TASK] Configure Travis for faster builds

* set sudo: false to allow Travis to use their new and faster
  container-based infrastructure
* cache the Composer packages between builds
Merge pull request #1556 from oliverklee/task/travis-performance

[TASK] Configure Travis for faster builds
Revert "[TASK] Configure Travis for faster builds"

This reverts commit ff3b69b06c75d6aace67a9a969f028b3eeadf308.
Merge branch 'master' of git://github.com/sebastianbergmann/phpunit