add clearTimeout polyfill to execjs server rendering

Author: xiaopow

lib/react_on_rails/server_rendering_pool/exec.rb

@@ -132,6 +132,10 @@ def execjs_timer_polyfills
 function setTimeout() {
   #{undefined_for_exec_js_logging('setTimeout')}
 }
+
+function clearTimeout() {
+  #{undefined_for_exec_js_logging('clearTimeout')}
+}
           JS
         end