Upgrade to RSpec3

RSpec 3.x introduces a `spec/rails_helper.rb` file
which contains all dependencies necessary to run specs that need Rails.

* Move Rails-specific things to rails_helper.rb.
* Require spec_helper.rb from rails_helper.rb.
* We do not need to require `spec_helper` manually.
  It is required for us in `.rspec`:
    --color
    --require spec_helper
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