version_field not respected on subclass

[c2c533c4f237]

I have a track_history with custom version_field set on a parent model, but the child model is still setting the default "version".

I'll reference this issue since this would be a fix as well: https://github.com/aq1018/mongoid-history/issues/53

[dblock]

@c2c533c4f237 It'd be great if you could turn this into a spec, I suspect we've fixed this.