Fix warnings in Ruby-2.4

[masutaka]

Before

$ bundle exec bin/github-nippou help
/Users/masutaka/src/github.com/masutaka/github-nippou/vendor/bundle/gems/concurrent-ruby-1.0.2/lib/concurrent/map.rb:206: warning: constant ::Fixnum is deprecated
/Users/masutaka/src/github.com/masutaka/github-nippou/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/xml_mini.rb:51: warning: constant ::Fixnum is deprecated
/Users/masutaka/src/github.com/masutaka/github-nippou/vendor/bundle/gems/activesupport-5.0.0/lib/active_support/xml_mini.rb:52: warning: constant ::Bignum is deprecated
Commands:
  github-nippou help [COMMAND]  # Describe available commands or one specific command
  github-nippou init            # Synchronize github-nippou settings on your gist
  github-nippou list            # Displays today's GitHub events formatted for Nippou
  github-nippou version         # Displays version

Options:
  s, [--since-date=SINCE_DATE]  # Retrieves GitHub user_events since the date
                                # Default: 20170730
  u, [--until-date=UNTIL_DATE]  # Retrieves GitHub user_events until the date
                                # Default: 20170730
  d, [--debug], [--no-debug]    # Debug mode

After

$ bundle exec bin/github-nippou help
Commands:
  github-nippou help [COMMAND]  # Describe available commands or one specific command
  github-nippou init            # Synchronize github-nippou settings on your gist
  github-nippou list            # Displays today's GitHub events formatted for Nippou
  github-nippou version         # Displays version

Options:
  s, [--since-date=SINCE_DATE]  # Retrieves GitHub user_events since the date
                                # Default: 20170730
  u, [--until-date=UNTIL_DATE]  # Retrieves GitHub user_events until the date
                                # Default: 20170730
  d, [--debug], [--no-debug]    # Debug mode