Vagrantfile: the gems are installed by bin/prepare_environment.bash

... so don't list the required gems redundantly in install-as-user as
well.
Vagrantfile: the virtualenv location depends on $DIRECTORY
Vagrantfile: make sure a native JSON gem is installed

Otherwise you get this warning (which seems to cause a failing return code)
from collectstatic:

  pipeline.exceptions.CompilerError: [WARNING] MultiJson is using the
  default adapter (ok_json).We recommend loading a different JSON library
  to improve performance.
Vagrantfile: yui-compressor must be installed for django-pipeline