Fix to add missing dependency to run generator spec individually

[tricknotes]

Without this require statement, the following error has thrown:

$ bundle exec rspec spec/react_on_rails/generators/generator_messages_spec.rb
An error occurred while loading ./spec/react_on_rails/generators/generator_messages_spec.rb.
Failure/Error:
  describe GeneratorMessages do
    it "has an empty messages array" do
      expect(GeneratorMessages.messages).to be_empty
    end

    it "has a method that can add errors" do
      GeneratorMessages.add_error "Test error"
      expect(GeneratorMessages.messages)
        .to match_array([GeneratorMessages.format_error("Test error")])
    end

NameError:
  uninitialized constant GeneratorMessages
# ./spec/react_on_rails/generators/generator_messages_spec.rb:5:in `<top (required)>'
No examples found.

Finished in 0.00026 seconds (files took 0.12917 seconds to load)
0 examples, 0 failures, 1 error occurred outside of examples

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This change is 

[justin808]

@robwise are you OK with this one? I really don't understand this change, given that tests were passing. Maybe without this fix, we can't run tests outside of the rake tasks. Any more info @tricknotes?

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[robwise]

I think what this is, is if you run the test quoted above individually (as opposed to as part of a suite), it fails because it has a dependency that loads when run as part of a suite, but not when run individually. Then this PR makes that dependency explicit?

[tricknotes]

Thanks, @robwise !
It is completely what I want to say.

[justin808]

@tricknotes Can you please add an entry to the CHANGELOG.md and I'll merge.

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Reviewed 1 of 1 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[tricknotes]

Thanks for your review, @justin808
I added an entry to the CHANGELOG.md and updated this PR.

[justin808]

Thanks @tricknotes! Merged!