Fixes a bug where wrong pkg.json was used

Currently the package.json file from generator-rb-js was being used in order to set the version of
grunt-jscs & grunt-contrib-jshint rather than getting the version for rb-js-linter.
So in here https://github.com/rockabox/rb-js-linter/blob/master/package.json#L21 we are specifying them as dependencies as the nature of the repo.