ci(cirrus): use cirrus-ci alongisde travis-ci

* Based on response from Cirrus CI:
  - fkorotkov/nginx-formula#1

.cirrus.yml

@@ -0,0 +1,10 @@
+docker_builder:
+  name: Test $INSTANCE
+  env:
+    matrix:
+      - INSTANCE: debian-9
+      - INSTANCE: debian-8
+      - INSTANCE: ubuntu-1804
+      - INSTANCE: ubuntu-1604
+  bundle_install_script: bundle install
+  verify_script: bundle exec kitchen verify $INSTANCE

.travis.yml

@@ -1,28 +1,8 @@
 stages:
-  - test
   - commitlint
   - name: release
     if: branch = master AND type != pull_request
 
-sudo: required
-cache: bundler
-language: ruby
-
-services:
-  - docker
-
-# Make sure the instances listed below match up with
-# the `platforms` defined in `kitchen.yml`
-env:
-  matrix:
-    - INSTANCE: debian-9
-    - INSTANCE: debian-8
-    - INSTANCE: ubuntu-1804
-    - INSTANCE: ubuntu-1604
-
-script:
-  - bundle exec kitchen verify ${INSTANCE}
-
 jobs:
   include:
     # Define the commitlint stage