Fix intermediate colors in gradients

Intermediate colors in gradients should be formatted as `color:offset`, not `offset:color`.

From the Raphael documentation: http://raphaeljs.com/reference.html#Element.attr

![image](https://f.cloud.github.com/assets/1017630/1948845/3b4d6384-80bb-11e3-8c73-7bfe5215fe1e.png)