Fix HTML5 Validation

The HTML validation of a document with Debugbar enabled gives this error `Element link is missing required attribute property.`

The line source of the error is link of stylesheet:

```
<link rel='stylesheet' type='text/css' href='http://localhost:8080/_debugbar/assets/stylesheets?v=1462974883'>
```

src/JavascriptRenderer.php

@@ -45,7 +45,7 @@ public function renderHead()
             'v' => $this->getModifiedTime('js')
        ]);
 
-        $html  = "<link rel='stylesheet' type='text/css' href='{$cssRoute}'>";
+        $html  = "<link rel='stylesheet' type='text/css' property='stylesheet' href='{$cssRoute}'>";
         $html .= "<script type='text/javascript' src='{$jsRoute}'></script>";
 
         if ($this->isJqueryNoConflictEnabled()) {