fix user_script query

because:

````
mysql> explain UPDATE `user_scripts` SET `assigned_at` = '2014-12-04 22:02:16' WHERE ((`user_id` IN (SELECT `student_user_id` FROM `followers` WHERE (`section_id` = '135906'))) AND (`script_id` = 26));
+----+--------------------+--------------+----------------+------------------------------------------------------------------+------------------------------------+---------+------+---------+------------------------------+
| id | select_type        | table        | type           | possible_keys                                                    | key                                | key_len | ref  | rows    | Extra                        |
+----+--------------------+--------------+----------------+------------------------------------------------------------------+------------------------------------+---------+------+---------+------------------------------+
|  1 | PRIMARY            | user_scripts | index          | NULL                                                             | PRIMARY                            | 4       | NULL | 2187913 | Using where; Using temporary |
|  2 | DEPENDENT SUBQUERY | followers    | index_subquery | index_followers_on_student_user_id,index_followers_on_section_id | index_followers_on_student_user_id | 4       | func |       1 | Using where                  |
+----+--------------------+--------------+----------------+------------------------------------------------------------------+------------------------------------+---------+------+---------+------------------------------+
2 rows in set (0.00 sec)

mysql> explain UPDATE `user_scripts` SET `assigned_at` = '2014-12-04 22:02:16' WHERE id = 1111111111;                                                                                          +----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table        | type  | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
|  1 | SIMPLE      | user_scripts | range | PRIMARY       | PRIMARY | 4       | const |    1 | Using where |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
1 row in set (0.00 sec)

mysql> explain select * from user_scripts where ((`user_id` IN (SELECT `student_user_id` FROM `followers` WHERE (`section_id` = '135906'))) AND (`script_id` = 26));
+----+-------------+--------------+--------+------------------------------------------------------------------+---------------------------------------------+---------+------------------------------------------------------+------+-----------------+
| id | select_type | table        | type   | possible_keys                                                    | key                                         | key_len | ref                                                  | rows | Extra           |
+----+-------------+--------------+--------+------------------------------------------------------------------+---------------------------------------------+---------+------------------------------------------------------+------+-----------------+
|  1 | SIMPLE      | followers    | ref    | index_followers_on_student_user_id,index_followers_on_section_id | index_followers_on_section_id               | 5       | const                                                |   30 | Start temporary |
|  1 | SIMPLE      | user_scripts | eq_ref | index_user_scripts_on_user_id_and_script_id                      | index_user_scripts_on_user_id_and_script_id | 8       | dashboard_production.followers.student_user_id,const |    1 | End temporary   |
+----+-------------+--------------+--------+------------------------------------------------------------------+---------------------------------------------+---------+------------------------------------------------------+------+-----------------+
2 rows in set (0.00 sec)
````