Fix readthedocs config

[thomthom]

Replace this text with a summary of the changes in your PR.
The more detailed you are, the better.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake. It executes all tests and RuboCop for itself, and generates the documentation.