Fix bug on deleting while using mysql 5.7

Move `DISTINCT` keyword to inner query to force the derived table to be
materialized in the query deleting hierarchy references.

Issue:
ClosureTree#190

Same issue in Rails:
rails/rails#19281

Related MySQL reference:
http://dev.mysql.com/doc/refman/5.7/en/upgrading-from-previous-series.html

Author: sanghyukkang

lib/closure_tree/hierarchy_maintenance.rb

@@ -94,8 +94,8 @@ def delete_hierarchy_references
         _ct.connection.execute <<-SQL.strip_heredoc
           DELETE FROM #{_ct.quoted_hierarchy_table_name}
           WHERE descendant_id IN (
-            SELECT DISTINCT descendant_id
-            FROM (SELECT descendant_id
+            SELECT descendant_id
+            FROM (SELECT DISTINCT descendant_id
               FROM #{_ct.quoted_hierarchy_table_name}
               WHERE ancestor_id = #{_ct.quote(id)}
                  OR descendant_id = #{_ct.quote(id)}