Support gecko's css gradient proposal

[Modernizr]

Reported here: http://twitter.com/atwork8_/statuses/3895252369

Per Faruk:
"Right now, the two CSS gradient specs are both mere proposals and both are under heavy scrutiny for changing and synchronizing. I want us to adjust the gradient check in the next release to support both WebKit and Mozilla's proposals, e.g. check for both types. Since Firefox only supports it as of 3.6a and later, but WebKit has had it since Safari 3.1, we'll do the Firefox check second."

[paulirish]

support mozilla css gradients proposal. closed by 118bf76