Test against go1.12

We need to start testing against go1.12. I don't know if we can drop
support for go1.10 yet, so I've left that untouched for now.

.gitlab-ci.yml

@@ -24,6 +24,10 @@ test using go 1.10:
 test using go 1.11:
   <<: *test_definition
 
+test using go 1.12:
+  image: golang:1.12
+  <<: *test_definition
+
 test:release:
   only:
     - tags