Using RubyGems to install Vagrant is deprecated, I guess.
Use wheezy-drupal box

Box comes with Drush all the packages we need.
We only manage apache vhosts and mysql databases with puppet now.
No longer install Drush with composer.
And finally, move scripts into the VM

We don't need to download selenium anymore, it's been replaced by PhantomJS
running in Webdriver mode.

Roll remaining deps (just `composer install` now) into build script.

Everything happens inside the VM now, get rid of calls to vagrant and simplify
the install script.