mixins/_progress.scss: Note that 0\0 media query hack is for IE9

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[skip validator]

scss/mixins/_progress.scss

@@ -9,6 +9,7 @@
     background-color: $color;
   }
 
+  // IE9
   @media screen and (min-width:0\0) {
     .progress-bar {
       background-color: $color;