Active Support 3.2 compatibility

[mnoack]

Thanks to changing the dependencies due to the fact that this gem breaks in ActiveSupport 3.2

My change (like a lot of other open source projects) will mean it's no longer compatible with active support 2.x, so that would need to be supported on another branch if it were to be supported.

I've forked and made what I "think" fixes the issue. It does pass "rake test" and "rake cucumber" but I'm not confident in my implementation as I haven't worked with class_attributes and inheritable_attributes functions and would appreciate someone having a look before I pull request.

One concern I have is will it work with inheritance type scenarios as I noticed some special inheritance_with_options calls which perhaps could be affected by my changes.

I also added some invalidate_cache! calls where I removed reset sources (since reseting sources is no longer needed when setting the source as class_attribute sets the attributes and the old write_inheritable_array was additive)

https://github.com/mnoack/rubigen/compare/drnic:master...active_support_3-2_compatibility

Thank You

[mnoack]

@drnic Can you have a quick check on my changes (they pass the tests but I need someone to review the changes who understands the RubiGen gem better)