Fix of ReferenceError: JSON is not defined

Runing the example on my machine:
mongo db --eval "var collection = 'collection'" variety.js

Fails with:
MongoDB shell version: 2.2.1
connecting to: db_name
Variety: A MongoDB Schema Analyzer
Version 1.2.3, released 01 September 2013
Wed Nov 20 11:30:49 ReferenceError: JSON is not defined variety.js:55
failed to load: variety.js

Apparently this was due to JSON object being uknown for some reason. Proposed change fixes it for me.

Thanks for the very handy tool.