Use the latest 11.x chef-client

After the update to omnibus 4 the `ffi-yajl` gem has been updated to a new major version which results in deprecation warnings when most `chef-server-ctl` commands are used.

```
the ffi-yajl and yajl-ruby gems have incompatible C libyajl libs and should not be loaded in the same Ruby VM
falling back to ffi which might work (or might not, no promises)
ffi-yajl/json_gem is deprecated, these monkeypatches will be dropped shortly
```

If we bump the chef-client version to the latest 11 stable version these deprecation warnings go away.