Pretty print JSON data

See travis-ci/travis-ci#1555

Before
```
$ curl -v -H "Accept: application/json" http://localhost:5000/repos/travis-ci/travis-core/builds.json > /dev/null
⋮
< Date: Fri, 15 Nov 2013 02:27:03 GMT
< Status: 200 OK
< Connection: close
< Content-Type: application/json;charset=utf-8
< Access-Control-Allow-Origin: *
< Access-Control-Allow-Credentials: true
< Access-Control-Expose-Headers: Content-Type, Cache-Control, Expires, Etag, Last-Modified
< X-Endpoint: Travis::Api::App::Endpoint::Repos
< X-Pattern: /:owner_name/:name/builds
< X-OAuth-Scopes: public
< X-Accepted-OAuth-Scopes: public
< Vary: Accept,Accept-Encoding
< Cache-Control: no-cache
< Content-Length: 2500
```

After
```
$ curl -v -H "Accept: application/json" http://localhost:5000/repos/travis-ci/travis-core/builds.json > /dev/null
⋮
< Date: Fri, 15 Nov 2013 02:29:01 GMT
< Status: 200 OK
< Connection: close
< Content-Type: application/json;charset=utf-8
< Access-Control-Allow-Origin: *
< Access-Control-Allow-Credentials: true
< Access-Control-Expose-Headers: Content-Type, Cache-Control, Expires, Etag, Last-Modified
< X-Endpoint: Travis::Api::App::Endpoint::Repos
< X-Pattern: /:owner_name/:name/builds
< X-OAuth-Scopes: public
< X-Accepted-OAuth-Scopes: public
< Vary: Accept,Accept-Encoding
< Cache-Control: no-cache
< Content-Length: 3125
```