v3.1.2.1

• Updated radial-gradient mixin and function to output new spec.
• Updated linear-gradient mixin and function to output new spec.
• Removed moz, ms, o vendor-prefixes from the linear-gradient() and radial-gradient() mixins and function.
• Updated background-image and background shorthand mixins to output new gradient specs.
• Added new backface-visibility mixin
• Removed unneeded vendor prefixes from transition and perspective mixins.
• Removed unnecessary ms vendor prefix from keyframes mixin
• Fixed bug in the prefixer mixin that was throwing unnecessary warnings. issue 205
• Added placeholder support for Firefox 19+