Test gitlab-workhorse against go1.5.4, 1.6.3 and 1.7.1

.gitlab-ci.yml

-image: golang:1.6.2
-
-test:
+.test_template: &test_definition
   script:
   - apt-get update -qq && apt-get install -y unzip bzip2
   - make test
 
+test:go1.5.4:
+  <<: *test_definition
+  image: golang:1.5.4
+
+test:go1.6.3:
+  <<: *test_definition
+  image: golang:1.6.3
+
+test:go1.7.1:
+  <<: *test_definition
+  image: golang:1.7.1