Fixed small button caret and moved less code to dropdowns

less/button-groups.less

@@ -141,8 +141,3 @@
   }
 }
 
-// Small button dropdowns
-.btn-small .caret {
-  margin-top: 4px;
-}
-

less/dropdowns.less

@@ -37,6 +37,10 @@
 .open.dropdown .caret {
   .opacity(100);
 }
+// Small button dropdowns
+.btn-small .caret {
+  margin-top: 6px;
+}
 // The dropdown menu (ul)
 .dropdown-menu {
   position: absolute;