update documentation about styled console logs

Firefox now support styled console logs since version 31 (actually on aurora) : https://hacks.mozilla.org/2014/05/editable-box-model-multiple-selection-sublime-text-keys-much-more-firefox-developer-tools-episode-31/

Author: wryk

Readme.md

@@ -91,7 +91,7 @@ setInterval(function(){
 #### Web Inspector Colors
 
   Colors are also enabled on "Web Inspectors" that understand the `%c` formatting
-  option. These are WebKit web inspectors, and the Firebug plugin for Firefox.
+  option. These are WebKit web inspectors, Firefox since version 31 and the Firebug plugin for Firefox.
   Colored output looks something like:
 
   ![](https://cloud.githubusercontent.com/assets/71256/3139768/b98c5fd8-e8ef-11e3-862a-f7253b6f47c6.png)