when using flexbox, don't add a clearfix

doing so fubars the grid column rounding (in safari at least) and causes layout problems

scss/mixins/_grid.scss

@@ -14,10 +14,11 @@
   @if $enable-flex {
     display: flex;
     flex-wrap: wrap;
+  } @else {
+    @include clearfix();
   }
   margin-left:  ($gutter / -2);
   margin-right: ($gutter / -2);
-  @include clearfix();
 }
 
 @mixin make-col($gutter: $grid-gutter-width) {