Add autocorrect to TimeZone cop

[dcluna]

Add autocorrect to TimeZone cop.
Uses .in_time_zone when possible and .zone when not.

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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.
• 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 rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[Drenmi]

Hi, @dcluna!

Please rebase this onto latest master and make sure your CHANGELOG entry ends up in the right place, and I think we're good to merge this. Thank you! 🙇

[bbatsov]

Thanks!