Do no render xray for CSS files

Example generated file (with SLIM):
```css
<!--XRAY START 749 /Users/example_user/projects/example_project/app/views/customizations/show.css.slim-->
#header { background-color: #ceefff; }
#header .nav > li > a { color: #fb0; }
<!--XRAY END 749-->
```
In it's current state this blocks the first rule from being applied.