recipes/system.rb: Replaced virtualbox with virtualbox-4.2.

As it is named in the Virtualbox apt rpeo. This should be
split up and install the corresponding sources.list as well.

Signed-off-by: Thorsten Fischer <thorsten@froschi.org>
recipes/system.rb: Removed heroku vom list of gems to install.

This is not installed via gems anymore. Instead, we need to install
the heroku-toolbelt apt package.

Note that this currently assumes that the corresponding sources.list is
in place.

Signed-off-by: Thorsten Fischer <thorsten@froschi.org>