Use latest gradient syntax for unprefixed version

The latest linear-gradient-syntax needs nother angles to work properly.
Browsers that render the latest un-prefixed syntax will fail otherwise.
Compass does not support the latest syntax. Please see this bug:
https://github.com/chriseppstein/compass/issues/965

This currently breaks the page in Chrome Canary.
Merge pull request #238 from h5bp/new-gradient-syntax

Use latest gradient syntax for unprefixed version