fix clear cache for prismic.io < 1.1.4

gistfile1.md

In app.js after routes:

app.route('/api/clear').get(routes.clearcache);

In routes/index.js

exports.clearcache = prismic.route(function(req, res, ctx) {
  ctx.api.apiCache.clear(function () {
    res.end(JSON.stringify({'cleared': 'ok'}, null, 2));
  });
});