ci(travis): test formula with Travis

.travis.yml

 ---
 dist: bionic
 stages:
+  - test
   - lint
   - 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: default-debian-10-develop-py3
+    # - INSTANCE: default-ubuntu-1804-develop-py3
+    # - INSTANCE: default-centos-7-develop-py3
+    # - INSTANCE: default-fedora-30-develop-py3
+    # - INSTANCE: default-opensuse-leap-15-develop-py3
+    # - INSTANCE: default-amazonlinux-2-develop-py2
+    # - INSTANCE: default-debian-9-2019-2-py3
+    - INSTANCE: default-ubuntu-1804-2019-2-py3
+    # - INSTANCE: default-centos-7-2019-2-py3
+    # - INSTANCE: default-fedora-30-2019-2-py3
+    # - INSTANCE: default-opensuse-leap-15-2019-2-py3
+    - INSTANCE: default-amazonlinux-2-2019-2-py2
+    # - INSTANCE: default-debian-9-2018-3-py2
+    # - INSTANCE: default-ubuntu-1604-2018-3-py2
+    # - INSTANCE: default-centos-7-2018-3-py2
+    - INSTANCE: default-fedora-29-2018-3-py2
+    - INSTANCE: default-opensuse-leap-15-2018-3-py2
+    # - INSTANCE: default-amazonlinux-2-2018-3-py2
+    # - INSTANCE: default-debian-8-2017-7-py2
+    # - INSTANCE: default-ubuntu-1604-2017-7-py2
+    - INSTANCE: centos6-centos-6-2017-7-py2
+    # - INSTANCE: default-fedora-29-2017-7-py2
+    # - INSTANCE: default-opensuse-leap-15-2017-7-py2
+    # - INSTANCE: default-amazonlinux-2-2017-7-py2
+
+script:
+  - bin/kitchen verify ${INSTANCE}
+
 jobs:
   include:
     # Define the `lint` stage (runs `yamllint` and `commitlint`)