Fix broken projects because of missing json-loader

[flybayer]

I ran into this problem when trying to use prisimic.io with next.js where I get this error:

ERROR in ./~/prismic.io/lib/requests.js
Module not found: Error: Cannot resolve module 'json-loader' in /Users/b/c/thefaithnet/node_modules/prismic.io/lib
 @ ./~/prismic.io/lib/requests.js 111:16-42 148:16-42

The offending line of code in prismic.io is this:

var pjson = require('../package.json');

I only get this error after installing next.

This happens because next is set to use json-loader in server/build/webpack.js, but next doesn't have the json-loader dependency defined.

This PR adds json-loader as a dependency.

My current workaround is to add json-loader as a dependency to my project.

[rauchg]

I think this is great. Thank you @flybayer