databases/mongo: require json for .to_json

The examples in the next block use .to_json, and this needs a `require
'json'` on top.
Merge pull request #41 from artagnon/patch-1

databases/mongo: require json for .to_json
Add a recipe about sinatra-partial gem.
Merge pull request #42 from patriciomacadden/sinatra-partial

Add a recipe about sinatra-partial gem.
explicit require form to support 1.8 and newer rubies, fixes #23