Convert pushstate to js

I feel like this function would be a bit easier to read if it looked like this rather:

```javascript
if ( this.isHistoryEnabled === undefined) {
  this.isHistoryEnabled = (window.history && window.history.pushState && window.history.replaceState && !navigator.userAgent.match(/((iPod|iPhone|iPad).+\bOS\s+[1-4]|WebApps\/.+CFNetwork)/));
}

return this.iHistoryEnabled;
```