Added allow_writeable_chroot config option

See the following issues:
http://askubuntu.com/questions/128180/vsftpd-stopped-working-after-update
http://blog.thefrontiergroup.com.au/2012/10/making-vsftpd-with-chrooted-users-work-again/

Adding the 'allow_writeable_chroot' solves them (for vsftpd > v3.0)