Only auto-merge to master and only when tests pass

.mergify.yml

 pull_request_rules:
 - name: auto-merge
   conditions:
+  - base=master
   - label=auto-merge
+  - status-success=continuous-integration/appveyor/branch
+  - status-success=continuous-integration/appveyor/pr
+  - status-success=continuous-integration/travis-ci/pr
+  - status-success=continuous-integration/travis-ci/push
+  - status-success=deploy/netlify
   actions:
     merge:
       method: merge