enable Dependabot v2 (#1134)

https://github.blog/2020-06-01-keep-all-your-packages-up-to-date-with-dependabot/

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "daily"