Update debian_install.sh

- spamassassin is disabled by default on debian.

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 spamaassasin

otherwise you get a lil message:

update-rc.d: using dependency based boot sequencing

and the init.rc isn't updated.
Merge pull request #11 from keshwarsingh/patch-1

Update debian_install.sh