Rename gradient mixins to match new naming convention

gradient-{horizontal=>x}[-three-colors]
gradient-{vertical=>y}[-three-colors]

scss/_carousel.scss

@@ -102,12 +102,12 @@
 
   // Set gradients for backgrounds
   &.left {
-    @include gradient-horizontal($start-color: rgba(0,0,0,.5), $end-color: rgba(0,0,0,.0001));
+    @include gradient-x($start-color: rgba(0,0,0,.5), $end-color: rgba(0,0,0,.0001));
   }
   &.right {
     right: 0;
     left: auto;
-    @include gradient-horizontal($start-color: rgba(0,0,0,.0001), $end-color: rgba(0,0,0,.5));
+    @include gradient-x($start-color: rgba(0,0,0,.0001), $end-color: rgba(0,0,0,.5));
   }
 
   // Hover/focus state

scss/mixins/_gradients.scss

@@ -4,7 +4,7 @@
 //
 // Creates two color stops, start and end, by specifying a color and position for each color stop.
 // Color stops are not available in IE9.
-@mixin gradient-horizontal($start-color: #555, $end-color: #333, $start-percent: 0%, $end-percent: 100%) {
+@mixin gradient-x($start-color: #555, $end-color: #333, $start-percent: 0%, $end-percent: 100%) {
   background-image: linear-gradient(to right, $start-color $start-percent, $end-color $end-percent);
   background-repeat: repeat-x;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#{ie-hex-str($start-color)}', endColorstr='#{ie-hex-str($end-color)}', GradientType=1); // IE9
@@ -14,7 +14,7 @@
 //
 // Creates two color stops, start and end, by specifying a color and position for each color stop.
 // Color stops are not available in IE9.
-@mixin gradient-vertical($start-color: #555, $end-color: #333, $start-percent: 0%, $end-percent: 100%) {
+@mixin gradient-y($start-color: #555, $end-color: #333, $start-percent: 0%, $end-percent: 100%) {
   background-image: linear-gradient(to bottom, $start-color $start-percent, $end-color $end-percent);
   background-repeat: repeat-x;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#{ie-hex-str($start-color)}', endColorstr='#{ie-hex-str($end-color)}', GradientType=0); // IE9
@@ -24,12 +24,12 @@
   background-repeat: repeat-x;
   background-image: linear-gradient($deg, $start-color, $end-color);
 }
-@mixin gradient-horizontal-three-colors($start-color: #00b3ee, $mid-color: #7a43b6, $color-stop: 50%, $end-color: #c3325f) {
+@mixin gradient-x-three-colors($start-color: #00b3ee, $mid-color: #7a43b6, $color-stop: 50%, $end-color: #c3325f) {
   background-image: linear-gradient(to right, $start-color, $mid-color $color-stop, $end-color);
   background-repeat: no-repeat;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#{ie-hex-str($start-color)}', endColorstr='#{ie-hex-str($end-color)}', GradientType=1); // IE9 gets no color-stop at all for proper fallback
 }
-@mixin gradient-vertical-three-colors($start-color: #00b3ee, $mid-color: #7a43b6, $color-stop: 50%, $end-color: #c3325f) {
+@mixin gradient-y-three-colors($start-color: #00b3ee, $mid-color: #7a43b6, $color-stop: 50%, $end-color: #c3325f) {
   background-image: linear-gradient($start-color, $mid-color $color-stop, $end-color);
   background-repeat: no-repeat;
   filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#{ie-hex-str($start-color)}', endColorstr='#{ie-hex-str($end-color)}', GradientType=0); // IE9 gets no color-stop at all for proper fallback