Go back to single colon pseudo elements.

This is for backwards compatibility with the browsers we support in v3.

less/mixins/clearfix.less

@@ -6,17 +6,17 @@
 //    Otherwise it causes space to appear at the top and bottom of elements
 //    that are clearfixed.
 // 2. The use of `table` rather than `block` is only necessary if using
-//    `::before` to contain the top-margins of child elements.
+//    `:before` to contain the top-margins of child elements.
 //
 // Source: http://nicolasgallagher.com/micro-clearfix-hack/
 
 .clearfix() {
-  &::before,
-  &::after {
+  &:before,
+  &:after {
     display: table; // 2
     content: " "; // 1
   }
-  &::after {
+  &:after {
     clear: both;
   }
 }