chore(.mergify.yml): don't wait for travis when [ci skip] is present (#…

…1789)

.mergify.yml

@@ -29,6 +29,20 @@ pull_request_rules:
         method: squash
         strict: smart
         strict_method: merge
+  # Allows PRs with [skip ci] and [ci skip] to be merged without waiting for Travis.
+  - name: automatic merge on CI success and review - push
+    conditions:
+      - title~=\[skip ci\]|\[ci skip\]
+      - "#changes-requested-reviews-by=0"
+      - base=master
+      - label=ready-to-merge
+      - approved-reviews-by=@leanprover-community/mathlib-maintainers
+    actions:
+     delete_head_branch: {}
+     merge:
+        method: squash
+        strict: smart
+        strict_method: merge
 
  # In practice this turns out to be really annoying.
  # - name: remove outdated reviews