enable gzip compression

A small change to enable gzip compression on the server.

I've deployed a version with the amendment to Heroku, you can see it here - https://lit-beach-8985.herokuapp.com/. You can see a comparison between the two by looking at the response headers of the following requests.

Original:

https://jsonp.nodejitsu.com/?callback=myCallback&url=http://jsonview.com/example.json

Amended:

https://lit-beach-8985.herokuapp.com/?callback=myCallback&url=http://jsonview.com/example.json