fix: missing subclass transitions

[ObscurePlatypoctopus]

Fixes #11

Caveat: The way it gets called changes with this, instead of creating an instance of the class with the state machine, call diagram on the class state machine instead.

[mihaimuntenas]

@Katee, @ObscurePlatypoctopus is correct and this PR fixes the issue.
Also, the calling convention seems to be "better" (the AASM DSL evaluates at class level) and does not break backward compatibility.

@ObscurePlatypoctopus welcome! And thank you for the PR!

Some quick notes/observations:

• bundler version update might be too high; the lowest version of supported aasm is 4.12, which requires ruby at minimum 1.9.3, thus it might depend on bundler from 1.x branch.
• you might want to also update the generate.rake to use the new calling convention (for completeness).

Again, thank you for the fix!

[ObscurePlatypoctopus]

@mihaimuntenas Thank you for the notes! I think the Ruby version I was using did not support the older bundler, but I don't think the upgrade was required after all so I removed it.

I also adjusted the rake task as per your notes, but I did not manage to verify it yet. I think there may be another issue in the Rakefile where

puts 'Missing `model` argument.' unless args[:model].present?

fails if not using rails, because string.present? is missing. I'm assuming aasm is usually used with rails so maybe this is not a big deal.

[mihaimuntenas]

@ObscurePlatypoctopus you are so right!
I actually use it in a Rails project, but I should come up with a fix for that - in the end, it is a Rake task, not a Rails task :) ...
And, as pointed out, there is no dependency on Rails.

[ObscurePlatypoctopus]

@Katee Is there anything this needs before it can be merged?

[adis-io]

@Katee Would be nice if it was merged