activemodel should be a dev dependency
Appraisable updates:

* always build rails-4 from gem
* build rails-edge from git
* updated gemfiles
make rails 4 required build, edge failable
Model.all spec changes for Rails 4

In Rails 3 Model.all returns an Array. In Rails 4 it returns an association proxy. Most of these changes are so that Rails 3 tests get an Array, and Rails 4 get a thing that quacks like an association proxy.