Make Rails/UnknownEnv cop aware of Rails.env == 'unknown_env'

[pocke]

Rails/UnknownEnv is not aware of Rails.env == 'unknown_env' style comparetion.

# It has a typo, but the cop does not detect any offense for the code!
if Rails.env == 'proudction'
end

This pull request adds the check to Rails/UnknownEnv cop.

Note

This pull request also change the offense message.
I removed ? from the environment name in the message.

- Unknown environment `proudction?`. Did you mean `production?`?
+ Unknown environment `proudction`. Did you mean `production`?

Because production? is not an environment name.

---

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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

Thanks!