ci(travis): run salt-lint during the Lint job

* Automated using https://github.com/myii/ssf-formula/pull/60
* Initially mentioned on Slack (@dafyddj):
  - https://freenode.logbot.info/saltstack-formulas/20191004#c2723464
* Basis for the `git ls-files` invocation (@sblaisot):
  - https://freenode.logbot.info/saltstack-formulas/20191004#c2724272

.salt-lint

@@ -0,0 +1,13 @@
+# -*- coding: utf-8 -*-
+# vim: ft=yaml
+---
+exclude_paths: []
+skip_list:
+  # Using `salt-lint` for linting other files as well, such as Jinja macros/templates
+  - 205  # Use ".sls" as a Salt State file extension
+  # Skipping `207` and `208` because `210` is sufficient, at least for the time-being
+  # I.e. Allows 3-digit unquoted codes to still be used, such as `644` and `755`
+  - 207  # File modes should always be encapsulated in quotation marks
+  - 208  # File modes should always contain a leading zero
+tags: []
+verbosity: 1

.travis.yml

@@ -27,9 +27,13 @@ jobs:
     # Run all of the linters in a single job
     - language: node_js
       node_js: lts/*
-      env: 'Lint: yamllint & commitlint'
+      env: 'Lint: salt-lint, yamllint & commitlint'
       before_install: skip
       script:
+        # Install and run `salt-lint`
+        - pip install --user salt-lint
+        - git ls-files | grep '\.sls$\|\.jinja$\|\.j2$\|\.tmpl$'
+                       | xargs -I {} salt-lint {}
         # Install and run `yamllint`
         # Need at least `v1.17.0` for the `yaml-files` setting
         - pip install --user yamllint>=1.17.0

.yamllint

@@ -17,6 +17,7 @@ yaml-files:
   # Default settings
   - '*.yaml'
   - '*.yml'
+  - .salt-lint
   - .yamllint
   # SaltStack Formulas additional settings
   - '*.example'