Add test:release job. Fail unless on a tag

.gitlab-ci.yml

@@ -15,3 +15,9 @@ test:go1.6.3:
 test:go1.7.1:
   <<: *test_definition
   image: golang:1.7.1
+
+test:release:
+  only:
+    - tags
+  script:
+    - git describe --exact-match