ci(travis): run rubocop during the Lint job

* Automated using myii/ssf-formula#60

.rubocop.yml

@@ -0,0 +1,9 @@
+# -*- coding: utf-8 -*-
+# vim: ft=yaml
+---
+# Increase from default of `80`
+# Based on https://github.com/PyCQA/flake8-bugbear#opinionated-warnings (`B950`)
+Metrics/LineLength:
+  Max: 88
+
+# Remaining offenses to be fixed, e.g. via. `rubocop --auto-gen-config`

.travis.yml

@@ -28,7 +28,7 @@ jobs:
 
     - language: node_js
       node_js: lts/*
-      env: 'Lint: salt-lint, yamllint & commitlint'
+      env: 'Lint: salt-lint, yamllint, rubocop & commitlint'
       before_install: skip
       script:
         # Install and run `salt-lint`
@@ -39,6 +39,9 @@ jobs:
         # Need at least `v1.17.0` for the `yaml-files` setting
         - pip install --user yamllint>=1.17.0
         - yamllint -s .
+        # Install and run `rubocop`
+        - gem install rubocop
+        - rubocop -d
         # Install and run `commitlint`
         - npm install @commitlint/config-conventional -D
         - npm install @commitlint/travis-cli -D