Update Sass vertical gradient mixin to latest version from Bootstrap.

As described in https://github.com/select2/select2/issues/3304#issuecomment-98395238, this updates the mixin to the latest stable version from Bootstrap Sass which adds Sass' http://sass-lang.com/documentation/Sass/Script/Functions.html#ie_hex_str-instance_method to ensure that colors are converted "into the format understood by IE filters".

dist/css/select2.css

@@ -276,7 +276,7 @@
   background-image: -o-linear-gradient(top, #ffffff 50%, #eeeeee 100%);
   background-image: linear-gradient(to bottom, #ffffff 50%, #eeeeee 100%);
   background-repeat: repeat-x;
-  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ffffff', endColorstr='#eeeeee', GradientType=0); }
+  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#FFFFFFFF', endColorstr='#FFEEEEEE', GradientType=0); }
   .select2-container--classic .select2-selection--single:focus {
     border: 1px solid #5897fb; }
   .select2-container--classic .select2-selection--single .select2-selection__rendered {
@@ -304,7 +304,7 @@
     background-image: -o-linear-gradient(top, #eeeeee 50%, #cccccc 100%);
     background-image: linear-gradient(to bottom, #eeeeee 50%, #cccccc 100%);
     background-repeat: repeat-x;
-    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#eeeeee', endColorstr='#cccccc', GradientType=0); }
+    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#FFEEEEEE', endColorstr='#FFCCCCCC', GradientType=0); }
     .select2-container--classic .select2-selection--single .select2-selection__arrow b {
       border-color: #888 transparent transparent transparent;
       border-style: solid;
@@ -342,7 +342,7 @@
   background-image: -o-linear-gradient(top, #ffffff 0%, #eeeeee 50%);
   background-image: linear-gradient(to bottom, #ffffff 0%, #eeeeee 50%);
   background-repeat: repeat-x;
-  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ffffff', endColorstr='#eeeeee', GradientType=0); }
+  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#FFFFFFFF', endColorstr='#FFEEEEEE', GradientType=0); }
 .select2-container--classic.select2-container--open.select2-container--below .select2-selection--single {
   border-bottom: none;
   border-bottom-left-radius: 0;
@@ -351,7 +351,7 @@
   background-image: -o-linear-gradient(top, #eeeeee 50%, #ffffff 100%);
   background-image: linear-gradient(to bottom, #eeeeee 50%, #ffffff 100%);
   background-repeat: repeat-x;
-  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#eeeeee', endColorstr='#ffffff', GradientType=0); }
+  filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#FFEEEEEE', endColorstr='#FFFFFFFF', GradientType=0); }
 .select2-container--classic .select2-selection--multiple {
   background-color: white;
   border: 1px solid #aaa;

src/scss/mixins/_gradients.scss

@@ -1,4 +1,4 @@
-// https://github.com/twbs/bootstrap-sass/blob/a416cb86a3ddc6906892b2c0472deea306989fea/vendor/assets/stylesheets/bootstrap/mixins/_gradients.scss
+// https://github.com/twbs/bootstrap-sass/blob/3.3-stable/assets/stylesheets/bootstrap/mixins/_gradients.scss#L17-L27
 
 // Vertical gradient, from top to bottom
 //
@@ -9,5 +9,5 @@
     background-image: -o-linear-gradient(top, $start-color $start-percent, $end-color $end-percent);  // Opera 12
     background-image: linear-gradient(to bottom, $start-color $start-percent, $end-color $end-percent); // Standard, IE10, Firefox 16+, Opera 12.10+, Safari 7+, Chrome 26+
     background-repeat: repeat-x;
-    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#{$start-color}', endColorstr='#{$end-color}', GradientType=0); // IE9 and down
+    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#{ie-hex-str($start-color)}', endColorstr='#{ie-hex-str($end-color)}', GradientType=0); // IE9 and down
 }