php artisan migrate : erorr : <code>$ php artisan migrate
Migration table created successfully.
In Connection.php line 647:
SQLSTATE[42000]: Syntax error or access violation: 1071 Specified key was too long; max key length is 767 bytes (SQL: alter table `users`
add unique `users_email_unique`(`email`))
In Connection.php line 449:
SQLSTATE[42000]: Syntax error or access violation: 1071 Specified key was too long; max key length is 767 bytes</code>

what's the problem?:|