Add dependabot.yml

Configure Dependabot with the most basic configuration for both
Ruby gems (via Bundler) and Github Actions.

Re: #71

.github/dependabot.yml

@@ -0,0 +1,15 @@
+version: 2
+
+updates:
+
+  # Maintain dependencies for GitHub Actions
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"
+
+  # Maintain dependencies for Ruby's Bundler
+  - package-ecosystem: "bundler"
+    directory: "/"
+    schedule:
+      interval: "daily"