fix bug in read-package-json.js, use readFileSync instead of require

[await-ovo]

when use npmCheck to get real-time dependencies info like codes below, it turns out that can't get updated info
it seems that can't read correct package.json info, because :

pkg = require(filename);

const npmCheck = require('npm-check');
 
npmCheck(options)
  .then(currentState => console.log(currentState.get('packages')));



// do something like yarn add package will change package.json



npmCheck(options)
  .then(currentState => console.log(currentState.get('packages')));