Asset pipeline

Hi,

I use your gem and it doesn't work, when i use image_tag function from rails helper.
I search some and request for better image hit wrong URL.

Request was send at
```
/assets/stat-icon-inactive-0842b16379ded9ddcc299912621f76bc@2x.png
```
but should 
```
/assets/stat-icon-inactive@2x-e8de7f87c2b9d08490575267a4c9eddc.png
```
after my change, request is sending at
```
/assets/stat-icon-inactive@2x.png
```

I want to write some test, but i haven't got any idea how to do it.