Make the ring class injectable

This adds the ability to pass a class into the options hash to be used as a ring.

The rationale behind it is that there are many other memcache libs using different hash ring logic, and being able to enforce that logic consistently throughout your applications shouldn't involve actually hacking dalli.

If you guys feel this would be a useful addition, I'll most likely add some documentation to this, just to be sure the use case for this is clear.