Re-enable RuboCop `Security/Open` warning

[ianfixes]

(Issue Summary / Feature Request)

lib/arduino_ci/arduino_downloader_windows.rb:30:7: C: Security/Open: The use of Kernel#open is a serious security risk.
      open(URI.parse(package_url), ssl_verify_mode: 0) do |url|
      ^^^^
lib/arduino_ci/arduino_downloader.rb:134:7: C: Security/Open: The use of Kernel#open is a serious security risk.
      open(package_url, ssl_verify_mode: 0, progress_proc: dot_printer) do |url|
      ^^^^