run go mod tidy in CI

Ensure that go.mod and go.sum are correct

.circleci/config.yml

@@ -37,6 +37,13 @@ jobs:
               git status --porcelain
               exit 1
             fi
+      - run:
+          name: verify go.mod and go.sum are correct
+          command: |
+            go mod tidy
+            git diff --quiet && exit 0
+            echo "please run 'go mod tidy' to ensure go.mod and go.sum are up to date"
+            exit 1
 
   go-test:
     executor: