Bump the default cost factor for bcrypt from 8 to 10

Bcrypt's current [default cost factor in PHP is 10](https://github.com/ircmaxell/password_compat/blob/master/lib/password.php#L40) but Laravel reduces this to 8. 

ircmaxell (the author of PHP's password hashing API) [recommends at least 9](http://blog.ircmaxell.com/2012/12/seven-ways-to-screw-up-bcrypt.html).