make tests compatible with Rails 4, and explicitly specify RSpec 2

When I clone a fresh copy of the repo and try to make changes, none of the tests work because the dummy app in `spec/dummy` still uses a bunch of config settings which were removed in Rails 4.

Also, running the specs using RSpec 3 throws a ton of deprecation warnings. The real solution is to upgrade the spec files to use RSpec 3's syntax, but for now I opted for the quick-fix of explicitly specifying RSpec 2 in the Gemfile to make sure it runs using that version.