Update slack_notifier.rb

Beautifying slack message.
Example:

    config.middleware.use ExceptionNotification::Rack,
                          slack: {
                            webhook_url: ENV['SLACK_WEB_HOOK'],
                            channel: '#channel-name',
                            username: 'Exception Notification',
                            icon_emoji: ':ghost:',
                            color: 'danger'
                          }