Add auto-releasing to the gitlab ci config.

.gitlab-ci.yml

@@ -1,5 +1,9 @@
 image: ruby:latest
 
+stages:
+  - test
+  - release
+
 before_script:
   - apt-get update && apt-get install wabt && rm -rf /var/cache/apt
   - bundle install
@@ -13,3 +17,16 @@ rubocop:
   stage: test
   script:
     - bundle exec rspec
+
+release:
+  stage: release
+  only:
+    - /^v[0-9]+\.[0-9]+\.[0-9]+$/
+  except:
+    - branches
+  script:
+    - gem build
+  artifacts:
+    name: "wag-$CI_COMMIT_TAG.gem"
+    paths:
+      - wag-*.gem