Make `Client.version` and `Client.server_version` properties

Current code in client.py

```python
def version(self):
"""Return client version."""
return local_version

def server_version(self):
"""Return the server version."""
return self.rpc_client.call("__router_server", "version")[1]
```

Should be

```python
@property
def version(self):
"""Return client version."""
return local_version

@property
def server_version(self):
"""Return the server version."""
return self.rpc_client.call("__router_server", "version")[1]
```

Or potentially `@cached_property`

This allows for the more idiomatic user experience of `client.version` rather than `client.version()`