Use frozen string literal in ci/

.rubocop.yml

@@ -92,6 +92,7 @@ Style/FrozenStringLiteralComment:
     - 'actionmailer/**/*'
     - 'actionview/**/*'
     - 'actionpack/**/*'
+    - 'ci/**/*'
     - 'guides/**/*'
     - 'tasks/**/*'
     - 'tools/**/*'

ci/travis.rb

 #!/usr/bin/env ruby
+# frozen_string_literal: true
+
 require "fileutils"
 include FileUtils