add a cleanup section and purge exim4

fixes #6

post-debootstrap-installer.sh

@@ -118,6 +118,11 @@ apt-get -y install \
     openssh-server
 
 
+### clean up some things
+apt-get -y purge exim4-base
+apt-get -y --purge autoremove
+
+
 ### dns
 
 apt-get -y install unbound