Fixes unmantained-activeadmin-plugins#1 bourbon border-radius mixin deprecation

Signed-off-by: Tommaso Visconti <tommaso.visconti@gmail.com>

Author: tommyblue

app/assets/stylesheets/active_admin/active_admin_dragonfly.css.sass

@@ -30,7 +30,7 @@
  padding: 5px
  background: white
  border: 1px solid #ccc
- +border-radius(2px)
+ border-radius: 2px
 
  &.component-url
  &> input[type="text"]