gitlab ci: build on more appropriate go versions

test against the standard versions in stretch and buster as well as the
latest go release on each of them.

.gitlab-ci.yml

@@ -16,15 +16,29 @@ before_script:
     - apt update
     - apt install --yes libpam0g-dev libgnutls28-dev libpam-wrapper pamtester oathtool postgresql xxd
 
-go1.8:
+go-1.7-stretch:
     image: golang:1.8-stretch
     script:
         - make test
     tags:
         - shared runner
 
-go1.9:
-    image: golang:1.9-stretch
+go-1.11-buster:
+    image: golang:1.11-buster
+    script:
+        - make test
+    tags:
+        - shared runner
+
+go-latest-stretch:
+    image: golang:stretch
+    script:
+        - make test
+    tags:
+        - shared runner
+
+go-latest-buster:
+    image: golang:buster
     script:
         - make test
     tags: