we can test on latest debian again

.gitlab-ci.yml

@@ -325,7 +325,7 @@ build_debian_package:
     - debian/create-debian-pkg.sh
 
 test_upgrade_debian_package:
-  image: debian:stretch
+  image: debian
   stage: test
   only:
     - tags
@@ -376,7 +376,7 @@ test_upgrade_debian_package:
      - test 1 = $(cat configuration.json| grep $version | wc -l)
 
 test_install_debian_package:
-  image: debian:stretch
+  image: debian
   stage: test
   only:
     - tags