fix circle not building tags

CircleCI recently skipped building tags by default, so we need to
explicitly enable it.

See https://circleci.com/docs/2.0/configuration-reference/#tags

.circleci/config.yml

@@ -23,3 +23,11 @@ jobs:
       - deploy:
           name: deploy
           command: .circleci/deploy.sh
+workflows:
+  version: 2
+  build:
+    jobs:
+      - build:
+          filters:
+            tags:
+              only: /.*/