Update prefixing mixins

[benknight]

The prefix mixins are doing a bit of unnecessary work:

1. transition no longer needs prefixing at all. http://caniuse.com/#search=transition
2. Transforms don't need -o- or -moz- prefixing: http://caniuse.com/#search=transform
3. Linear gradients no longer need prefixing really, except maybe -webkit- for super deep mobile browser support? http://caniuse.com/#search=transform

[benknight]

Happy to submit a pull request if authors agree. :)

[cshold]

Agreed, I'll 👍 a PR. I was actually thinking of removing the gradient mixin. It's not used anywhere, and I've very rarely been wanting one while developing.

[graygilmore]

Looks like this has been taken care of? 3ab33cb

[cshold]

Fixed via #410