Enable Dependabot for GitHub Actions (#416)

Author: andyw8

.github/dependabot.yml

@@ -15,3 +15,13 @@ updates:
         update-types:
           - "minor"
           - "patch"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    reviewers:
+      - "Shopify/ruby-dev-exp"
+    labels:
+      - "chore"
+      - "dependencies"
+      - "gh-actions"