Generate rails_helper, not spec_helper

RSpec 3.x introduces a `spec/rails_helper.rb` file which contains all
dependencies necessary to run specs that need Rails.

https://www.relishapp.com/rspec/rspec-rails/docs/upgrade#default-helper-files

Achieves the goal of "Documenting Explicit Dependencies Through Tests":

http://robots.thoughtbot.com/document-explicit-dependencies-through-tests

* Move Rails-specific things to rails_helper.rb.
* Require spec_helper.rb from rails_helper.rb.
* Remove duplication across files.
* We do not need to require "spec_helper" manually.
  It is required for us in `.rspec`:

```
--color
--warnings
--require spec_helper
```
It's included in `rails_helper.rb` by default in rspec-rails.

https://www.relishapp.com/rspec/rspec-rails/docs/directory-structure