benchmark: require better performance during benchmark for fast browsers

Browsers got better at the benchmark without being better in the real
life. Make benchmark more difficult. Related to #47.

app/scripts/receiver/benchmark.js

@@ -135,7 +135,7 @@ module.exports = (function(window) {
         // Record the number of frames displayed
         var fps = frames / duration * 1000;
         console.log('[Dashkiosk] Benchmark: ' + fps + ' fps');
-        if (fps > 5) {
+        if (fps > 25) {
           document.body.classList.add('dk-fast-browser');
         }