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.
Merge pull request #11 from opscode-cookbooks/jtimberman/default-v2-version

Set a default version of chef-vault