Don't add the ruby label to dependabot pull requests

The "ruby" label was redundant because most our dependencies are ruby
gems. We're still keeping the "javascript" dependencies for npm
dependencies, though.

.github/dependabot.yml

@@ -5,6 +5,8 @@ updates:
   schedule:
     interval: monthly
   open-pull-requests-limit: 50
+  labels:
+    - "dependencies"
 - package-ecosystem: npm
   directory: "/"
   schedule: