AutoMarshallingMap doesn't have a #empty? method.

[Mike]

~/.rvm/gems/ruby-2.4.0/gems/markov_chain_chat_bot-0.1.7/lib/markov_chain_chat_bot.rb:25:in `initialize': undefined method `empty?' for #<AutoMarshallingMap:0x00000001cf7dc0> (NoMethodError)
        from ~/.rvm/gems/ruby-2.4.0/gems/markov_chain_chat_bot-0.1.7/lib/markov_chain_chat_bot.rb:20:in `new'
        from ~/.rvm/gems/ruby-2.4.0/gems/markov_chain_chat_bot-0.1.7/lib/markov_chain_chat_bot.rb:20:in `from'
        from ./train.rb:25:in `<main>'

I was attempting to do the key-value storage examples, and it crashed.

A potential fix would be to add a #empty? method to AutoMarshallingMap that simply returns false.
Or at least that seems to have worked in my case.

If you like I can submit a pull request with the fix. I figured I ought to make an issue first, because I don't know the potential repercussions.