Realm 0.85 has primary key support and does not allow it to change

This updates Realm-JSON to use Realm 0.85. 

As this Realm version supports primary keys I needed to add a control when adding data in `-mc_setValuesFromJSONDictionary:inRealm:` so that it's not trying to write new data for a primary key.

I've not checked my solution thoroughly, but it's a suggestion as to what needs to be done to solve this problem.