dependabot: check for github actions and bundler

dependabot will then watch for outdated gems or github actions and
create PRs to update them.

.github/dependabot.yml

@@ -0,0 +1,17 @@
+version: 2
+updates:
+  # raise PRs for gem updates
+  - package-ecosystem: bundler
+    directory: "/"
+    schedule:
+      interval: daily
+      time: "13:00"
+    open-pull-requests-limit: 10
+
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: github-actions
+    directory: "/"
+    schedule:
+      interval: daily
+      time: "13:00"
+    open-pull-requests-limit: 10