Require `rails_helper` instead of `spec_helper` on rspec

[tricknotes]

rails_helper is an actual entry point of specs. This change allows developers to running single spec like the following:

$ bundle exec rspec spec/models/letter_opener_web/letter_spec.rb

Without this change, the following error will be occurred:

$ bundle exec rspec spec/models/letter_opener_web/letter_spec.rb

An error occurred while loading ./spec/models/letter_opener_web/letter_spec.rb.
Failure/Error:
  RSpec.describe LetterOpenerWeb::Letter do
    let(:location) { Pathname.new(__dir__).join('..', '..', 'tmp').cleanpath }

...

NameError:
  uninitialized constant LetterOpenerWeb::Letter

[fgrehm]

Thank you!