Update backports url

.gitlab-ci.yml

@@ -51,9 +51,9 @@ cert-renewal:
   variables:
     CERTBOT_RENEWAL_GIT_TOKEN: $CERTBOT_RENEWAL_GIT_TOKEN
   script:
-    - echo "deb http://ftp.debian.org/debian jessie-backports main" >> /etc/apt/sources.list
+    - echo "deb http://deb.debian.org/debian stretch-backports main" >> /etc/apt/sources.list
     - apt-get update
-    - apt-get install certbot -t jessie-backports -y
+    - apt-get install certbot -t stretch-backports -y
     - apt-get install git curl -y
     - export PATH=$PATH:$CI_PROJECT_DIR
     - git config --global user.name $GITLAB_USER_LOGIN