Fixes invalid hex codes generated from RGB values

If you include a color declaration that looks like this:

    background-color: rgb(256,110,200);

It makes the background [a pretty pink color](http://jsbin.com/epenaq). If you ran the CSS minifier against it, it would munge the rule to:

    background-color: #1006ec8;

Which is an invalid color and [doesn't look like anything](http://jsbin.com/idazef). This normalizes the apparent behavior by maxing out any rgb values at 255 before minification.