ci(travis): reduce matrix down to 6 instances (ref: #118)

* The selected 6 in a "tree"-shaped matrix:
  - 2019.2 (py3): debian-9   centos7   opensuse-leap-15
  - 2018.3 (py2):     fedora-29   ubuntu-1604
  - 2017.7 (py2):            centos6
* Covers each `os`, each Salt version and both Python versions

.travis.yml

@@ -13,27 +13,33 @@ services:
 
 # Make sure the instances listed below match up with
 # the `platforms` defined in `kitchen.yml`
+# NOTE: To minimise disruption across the whole org, only select five instances
+#       to run and leave the rest commented out
+#       The recommended matrix is shown below
+#       Allow `kitchen.yml` to contain all of the platforms, to allow
+#       comprehensive local testing
+#       Ref: https://github.com/saltstack-formulas/template-formula/issues/118
 env:
   matrix:
     - INSTANCE: default-debian-9-2019-2-py3
-    - INSTANCE: default-ubuntu-1804-2019-2-py3
+    # - 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-leap-42`
     # Ref: https://github.com/netmanagers/salt-image-builder/issues/2
     # - INSTANCE: default-opensuse-leap-15-2018-3-py2
-    - INSTANCE: default-opensuse-leap-42-2018-3-py2
-    - INSTANCE: default-debian-8-2017-7-py2
-    - INSTANCE: default-ubuntu-1604-2017-7-py2
+    # - INSTANCE: default-opensuse-leap-42-2018-3-py2
+    # - INSTANCE: default-debian-8-2017-7-py2
+    # - INSTANCE: default-ubuntu-1604-2017-7-py2
     # TODO: Enable after improving the formula to work with other than `systemd`
     - 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}