Switch from AutoMerge GH Action to Mergify.io

Author: stephendolan

.github/mergify.yml

@@ -0,0 +1,21 @@
+# Provides automatic merging of pull requests based on specific rules.
+#
+# See: https://docs.mergify.io/configuration.html
+
+pull_request_rules:
+  - name: Automatically merge Dependabot pull requests
+    conditions:
+      - author~=^dependabot(|-preview)\[bot\]$
+      - check-success=CheckFormat
+      - check-success=RunSpecs
+    actions:
+      merge:
+        method: squash
+
+  - name: When a dependency conflict emerges, ask Dependabot to rebase its update
+    conditions:
+      - conflict
+      - author~=^dependabot(|-preview)\[bot\]$
+    actions:
+      comment:
+        message: "@dependabot rebase"