More Rails 4 fixes

First and second commits are required for app to load on Rails 4. Second commit breaks on 3.2, so fourth commit fixes it by doing the string manipulation ourselves instead of having ActiveModel::Name do it.

Third commit is a stowaway. Running `rails g active_admin:install` on a codebase that already had AdminUser and Devise setup added the ActiveAdmin routing configuration twice, which doesn't work.

I can't get the specs to run on Rails 4 (missing "posts" table), so, um, sorry for lack of testing.