Configure dependabot for github-actions

Author: toshimaru

.github/dependabot.yml

@@ -1,7 +1,14 @@
 version: 2
 updates:
-- package-ecosystem: bundler
-  directory: "/"
-  schedule:
-    interval: weekly
-  open-pull-requests-limit: 10
+  - package-ecosystem: bundler
+    directory: "/"
+    schedule:
+      interval: weekly
+    reviewers:
+      - toshimaru
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: weekly
+    reviewers:
+      - toshimaru