Add requirement for Chef client > 11.10

Chef client < 11.10 breaks with

```
Gem::LoadError
--------------
Unable to activate httparty-0.13.0, because json-1.7.7 conflicts with json (~> 1.8)
```

See https://tickets.opscode.com/browse/CHEF-4632 and http://docs.opscode.com/release_notes.html.