Set a default version of chef-vault

The version of chef-vault needs to be set to a sane default, where "sane" is
defined by:

    A version of chef-vault that has the API we use in the helpers.

The current release is 2.2.4, which will be installed on new systems. However,
if a system had e.g. `1.2.5`, it wouldn't have the ChefVault::Item class used
by the `chef_vault_item` helper.
Scumbag me does this here instead of using #10 :ice_cream: