Rails 4.0.0.beta1 compatibility

- Update dependencies
- Get rid of observers tests (I tried to use rails-observer gems but it was a mess)
- Update deprecated calls like #scope or #default_scope without block
- Get rid of attr_protected call and tests
- Get rid of AR#destroy! NoMethodError test

The last one cause issue because in AR 4 #destroy! bang is defined.
It's the same as #destroy but raise an AR::RecordNotFound if the record
is not present in database.

It may be smart to find another name for this method. But for now it just works.