Use json instead of ruby with eval

Currently, the gem uses the "ruby" response type from solr and evals the result to get a hash.

We should instead ask for json and parse the json. It is easier to tell if the response is bad and is more secure than executing arbitrary ruby from a source that could be compromised.

Also, we should probably raise an exception if bad results are specified (perhaps the solr server is crashing). You should rescue this in your application to handle appropriately.