Update debian_install.sh

- SpamAssassin is disabled by default on Debian 7, The ISPConfig 3 setup uses amavisd which loads the SpamAssassin filter library internally..

As of 7.1 (Debian Wheezy), dependency based boot is used by default, instead of:

update-rc.d -f spamassassin remove

you would use:

insserv -rf spamassassin

otherwise you get a lil message:

update-rc.d: using dependency based boot sequencing

and the init.rc isn't updated.