Update 2012_12_06_225988_migration_cartalyst_sentry_install_throttle.php

This fixes an issue in which you could potentially see when login in from different IP's.

1062 Duplicate entry '1' for key 'throttle_user_id_unique' (SQL: insert into `throttle` (`user_id`, `ip_address`) values (?, ?)) (Bindings: array ( 0 => '1', 1 => 'xxx.xx.xx.xx', ))

It also matches to meet the schema, which is not unique.
sentry/schema/mysql.sql