Don't use FreezeTime cop in Rails <5.2

[DRBragg]

freeze_time wasn't added until Rails 5.2, so running this cop against a Rails <= 5.1 app returns inaccurate offenses.

I have:

• Added specs and confirmed failures.
• Set minimum_target_rails_version to 5.2 in the Rails/FreezeTime Cop.
• Reran specs and confirmed no failures.
• Added a changelog entry

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Can you add the PR address?

Suggested change

	* Fix `Rails/FreezeTime` running against Rails < 5.2 apps. ([@DRBragg][])
	* [#918](https://github.com/rubocop/rubocop-rails/pull/918): Fix `Rails/FreezeTime` running against Rails < 5.2 apps. ([@DRBragg][])

[DRBragg]

Yes, absolutely! I apologize that I didn't have it in there originally. When I read the Contributing guide section on the changelog I assumed it only applied to issue links

[andyw8]

@koic Is there a policy on which Rails versions rubocop-rails should support?

[koic]

@andyw8 Rails 4.2 and above now:
https://github.com/rubocop/rubocop-rails#compatibility

But honestly strongly conservative about supporting older versions. This patch is acceptable because it is a minor fix to existing feature and is not difficult to maintain.

I haven't decided yet, but in the future the following may hint at maintenance policy:
https://guides.rubyonrails.org/maintenance_policy.html

[koic]

@DRBragg CI is failing. Can you take a look?

[DRBragg]

I'm sorry @koic, I missed a linter error. Please forgive the oversight...

[koic]

Never mind! Thank you for the fixing!