Merge pull request activeadmin#208 from emzeq/master

Small IE CSS Fix

Author: mattvague

app/assets/stylesheets/active_admin/_base.css.scss

@@ -329,7 +329,7 @@ table.dashboard {
   }
   th {
     @include no-shadow;
-    background: none;
+    @include no-gradient;
     width: 150px;
     font-size: 0.9em;
     padding-left: 0;

app/assets/stylesheets/active_admin/mixins/_gradients.css.scss

@@ -19,3 +19,11 @@ $secondary-gradient-stop: #dfe1e2;
 @mixin secondary-gradient { 
   @include gradient($secondary-gradient-start, $secondary-gradient-stop);
 }
+
+@mixin no-gradient {
+  background: none;
+  // IE 6 & 7
+  filter: progid:DXImageTransform.Microsoft.gradient(enabled = false);
+  // IE 8
+  -ms-filter: "progid:DXImageTransform.Microsoft.gradient(enabled=false)";
+}