Previously carthage.vault.VaultConfigPlugin called the
hvac.Client.read function which uses the KVv1 endpoint.
KVv2 or KVv1 secrets may now be specified in the config section as follows:
```yaml
config:
some_v1_secret: "{vault:path/to/secret:field}"
some_v2_secret: "{vault:v2:some_mountpoint:path/to/v2_secret:field}"
```