Fix progress bar colors in IE (#1334)

sass/elements/progress.sass

@@ -18,6 +18,8 @@ $progress-value-background-color: $text !default
     background-color: $progress-value-background-color
   &::-moz-progress-bar
     background-color: $progress-value-background-color
+  &::-ms-fill
+    background-color: $progress-value-background-color
   // Colors
   @each $name, $pair in $colors
     $color: nth($pair, 1)
@@ -26,6 +28,8 @@ $progress-value-background-color: $text !default
         background-color: $color
       &::-moz-progress-bar
         background-color: $color
+      &::-ms-fill
+        background-color: $color
   // Sizes
   &.is-small
     height: $size-small