Add gomodTidy to postUpdateOptions in renovate.json (#50)

This will lead to `go mod tidy` being run after go module updates to
purge unused verison hashes from go.sum

See https://docs.renovatebot.com/configuration-options/#postupdateoptions
for details.

renovate.json

@@ -1,5 +1,8 @@
 {
   "extends": [
     "config:base"
+  ],
+  "postUpdateOptions": [
+    "gomodTidy"
   ]
 }