RuboCop 0.49.1; rake task; linting

[olleolleolle]

This PR introduces responds_to_missing? RuboCop warning fixes.

• default Rake task: include RuboCop
• RuboCop: update TODO file
• RuboCop: display name and URL for style warnings, to make them more actionable
• Gemfile: pin to exact version of RuboCop
• Appveyor: Styleguide rule on crlf in Git.

[olleolleolle]

(I had made a typo in the RuboCop configuration file which made it not exclude the vendor/**/* pattern. Now it ought to run to completion.)

[olleolleolle]

@sickill Ha!