Added Foreman as a dependency.

[jamespearson]

[justin808]

@fearmediocrity Most devs have foreman installed globally. Any reason we should add to the gemspec? for contributors?

I could see maybe adding this to what's generated. Maybe it's already there.

[jamespearson]

I added it because if you follow your instructions I had to manually. ( I was using a sandboxed app to try it out).

If a dev already uses it then it will be an already satisfied dependency, if not it will save a minute of head scratching for the beginner coming from React to Rails.

[justin808]

Thanks.