ci(travis): reduce matrix down to 6 instances

* Ref: saltstack-formulas/template-formula#121

Author: myii

Diff for: .travis.yml

@@ -22,21 +22,21 @@ env:
     # TODO: Re-enable this once the `systemd` service can be restarted reliably
     # - INSTANCE: default-ubuntu-1804-2019-2-py3
     - INSTANCE: default-centos-7-2019-2-py3
-    - INSTANCE: default-fedora-29-2019-2-py3
+    # - INSTANCE: default-fedora-29-2019-2-py3
     - INSTANCE: default-opensuse-leap-15-2019-2-py3
-    - INSTANCE: default-debian-9-2018-3-py2
+    # - INSTANCE: default-debian-9-2018-3-py2
     - INSTANCE: default-ubuntu-1604-2018-3-py2
-    - INSTANCE: default-centos-7-2018-3-py2
+    # - INSTANCE: default-centos-7-2018-3-py2
     - INSTANCE: default-fedora-29-2018-3-py2
     # TODO: Use this when fixed instead of `opensuse-423`
     # NOTE: Having to temporarily disable `opensuse-423` due to slow mirror
     # - INSTANCE: default-opensuse-leap-15-2018-3-py2
     # - INSTANCE: default-opensuse-423-2018-3-py2
-    - INSTANCE: default-debian-8-2017-7-py2
-    - INSTANCE: default-ubuntu-1604-2017-7-py2
+    # - INSTANCE: default-debian-8-2017-7-py2
+    # - INSTANCE: default-ubuntu-1604-2017-7-py2
     - INSTANCE: default-centos-6-2017-7-py2
-    - INSTANCE: default-fedora-28-2017-7-py2
-    - INSTANCE: default-opensuse-leap-42-2017-7-py2
+    # - INSTANCE: default-fedora-28-2017-7-py2
+    # - INSTANCE: default-opensuse-leap-42-2017-7-py2
 
 script:
   - bundle exec kitchen verify ${INSTANCE}