Add and appy RSpec/Rails/TravelAround rubocop rule

This rule was added in rubocop-rspec 2.19.0.

When freezing time in a test, `travel_back` is called automatically when
the test finishes, so we can do it in a `before` block instead of an
`around` block.

Note this rule didn't detect our usage of `freeze_time` because we were
using it on cops with a certain tag, but I expect the rule to be able to
detect this usage in the future.

.rubocop.yml

@@ -561,6 +561,9 @@ RSpec/Rails/HaveHttpStatus:
 RSpec/Rails/InferredSpecType:
   Enabled: true
 
+RSpec/Rails/TravelAround:
+  Enabled: true
+
 RSpec/RepeatedExample:
   Enabled: true