Use new container-based env and caching

Let's see how much the speed will be improved in the new travis environment with caching enabled.
no, I've added the base version to composer.json (see your below comment). That's because the `vendor` directory is now cached (which improves composer's speed). This env variable now defaults to "no", meaning just install the one in composer.json. It only requires another version when this variable is changed to something else than "no".

This is done because Composer had lots of weird issues since there was a `symfony/symfony` package in the cached vendor, but not in `composer.json`. It said some package required "sensio/generator-bundle: 2.6", while `symfony/symfony: 2.3.x` required `sensio/generator-bundle: 2.3`.

Now the package is in the composer.json, Composer now understands that we want the package in another version.