Makes rubocop happy

this commit introduces a few changes to make rubocop happy:
  - disable the name checking on filename (didn't wanted to change the
    dashed filenames for backward compat)
  - replace %w{} literal notation to %w()
  - use File.exist? instead of deprecated File.exists?

Signed-off-by: Christophe Courtaut <christophe.courtaut@gmail.com>