Turn on automerge with mergify (#56)

.mergify.yml

@@ -0,0 +1,23 @@
+pull_request_rules:
+  - name: automatic merge on CI success
+    conditions:
+      - status-success=continuous-integration/travis-ci/pr
+      - label=automerge
+    actions:
+      merge:
+        method: rebase
+        strict: true
+  - name: remove automerge label on CI failure
+    conditions:
+      - status-failure=continuous-integration/travis-ci/pr
+      - label=automerge
+    actions:
+      label:
+        remove:
+          - automerge
+  - name: remove outdated reviews
+    conditions:
+      - base=master
+    actions:
+      dismiss_reviews:
+        changes_requested: true