Ensure that the backports version of certbot is installed

src/freedombone-addcert

@@ -218,7 +218,7 @@ function add_cert_letsencrypt {
     fi
 
     if [ ! -f /usr/bin/certbot ]; then
-        apt-get -yq install certbot
+        apt-get -yq -t stretch-backports install certbot
         groupadd ssl-cert
         if [ ! -f /usr/bin/certbot ]; then
             echo $'LetsEncrypt certbot failed to install'

src/freedombone-upgrade

@@ -93,6 +93,7 @@ if [ -d $PROJECT_DIR ]; then
             exit 453536
         fi
 
+        apt-get -yq -t stretch-backports install certbot
         email_install_tls
         defrag_filesystem
     fi