[Compliance] Make encryption_key optional until it is supported?

[Mr0grog]

The compliance readme currently lists the encryption_key config property as “not working yet,” but it is still required in the configuration validation routine: https://github.com/stellar/bridge-server/blob/master/src/github.com/stellar/gateway/compliance/config/config.go#L59-L62

Should it be optional until support for it is implemented? If not, we should probably at least update the readme to clarify that you still have to specify it (and it has to be valid).