Fix json serialization and deserialization

Update to work with json gems released in the last 8 months. rspec tests still need updating to work with JSON 1.5.5 and not assuming that JSON.parse() will directly create a DataBagItem with .raw_data attribute. 

This fixes problems with chef-solo clients using JSON 1.5.5+ reading the created data bags.

https://github.com/thbishop/knife-solo_data_bag/issues/11