Upgrade to Rails 4

In the book you're using Rails 4, but this code is outdated. As I was getting errors running my code, I downloaded these examples, but they were in Rails 3.2! This PR updates the code to Rails 4 and fixes the failing specs.

Using subscribem.root_url instead of root_path throws an error when running the specs:
```
Failures:

  1) Accounts creating an account
     Failure/Error: visit subscribem.root_url
     Apartment::SchemaNotFound:
       One of the following schema(s) is invalid: public
     # ./spec/features/accounts/sign_up_spec.rb:5:in `block (2 levels) in <top (required)>'
```