Fix `rake state_machine:draw` on namespace collisions 

This PR fixes a problem of the `state_machine:draw` with namespace collisions.

Given:

```
class User; end # no state_machine
class Admin::User; end #includes state_machine
```

Running  `rake state_machine:draw CLASS=Admin::User` will fail, as class resolves to `User` which doesn't have a statemachine defined.  This PR fixes the klass resolving by disabling inheritance lookup of [`const_defined?`](
http://www.ruby-doc.org/core-1.9.3/Module.html#method-i-const_defined-3F). Added a fallback for older ruby version where this feature dosen't exist.