ci: merge travis matrix, add salt-lint & rubocop to lint job

* Semi-automated using https://github.com/myii/ssf-formula/pull/60

Gemfile

@@ -1,6 +1,7 @@
-source "https://rubygems.org"
+# frozen_string_literal: true
+
+source 'https://rubygems.org'
 
 gem 'kitchen-docker', '>= 2.9'
-gem 'kitchen-salt', '>= 0.6.0'
 gem 'kitchen-inspec', '>= 1.1'
-
+gem 'kitchen-salt', '>= 0.6.0'