🐛 Update linear-gradient to new syntax

Updates the generic linear-gradient rule to use the new `to {x}` syntax. The default direction for linear-gradients is `to bottom` so that is what I used in this case.

This removes the autoprefixer warnings we see in Framework to update our linear-gradient rules to adhere to the new standard.

In theory, this mixin doesn’t even need to be used as autoprefixer handles all of these vendor prefixes for us :)

css-dev/burf-base/_mixins.scss

@@ -166,7 +166,7 @@
 		background-image: -moz-linear-gradient(top, $from-color, $to-color); // FF3.6
 		background-image: -ms-linear-gradient(top, $from-color, $to-color); // IE10
 		background-image: -o-linear-gradient(top, $from-color, $to-color); // Opera 11.10+
-		background-image: linear-gradient(top, $from-color, $to-color);
+		background-image: linear-gradient(to bottom, $from-color, $to-color);
 		filter: progid:DXImageTransform.Microsoft.gradient(GradientType=0,StartColorStr='#{$ie-from}', EndColorStr='#{$ie-to}');
 	} @else {
 		@error 'The linear gradient mixin requires two valid colors or variables which hold colors. \a Example usage: @include linear-gradient( $from-color, $to-color);';