Fix a false negative for RSpecRails/TravelAround cop when passed as a proc to a travel method

[ydah]

Fixes: #49

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• [-] Added the new cop to config/default.yml.
• [-] The cop is configured as Enabled: pending in config/default.yml.
• [-] The cop documents examples of good and bad code.
• [-] The tests assert both that bad code is reported and that good code is not reported.
• [-] Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

• [-] Set VersionChanged: "<<next>>" in config/default.yml.

[bquorning]

Why do we prefer before over around?

[ydah]

@bquorning this cops motivation is here:

Since Rails 5.2, travel_back is called automatically, so you can easily do that by just calling it in before instead of using around with nested blocks.

• Remove time stubs after each test rails/rails#29860

refs: rubocop/rubocop-rspec#1503