Update sign_up_spec to get tests passing

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)>'
```