[Rails5] expected "2018-05-07 13:53:08 UTC" to be an instance of ActiveSupport::TimeWithZone

Note: add rails5 to a branch name to trigger rails5 specific CI jobs.

https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/75037360

  1) RedisCacheable#cast_value_from_cache with runner contacted_at converts cache string to appropriate type
     Failure/Error: expect(subject).to be_an_instance_of(ActiveSupport::TimeWithZone)
       expected "2018-05-07 13:53:08 UTC" to be an instance of ActiveSupport::TimeWithZone
     # ./spec/models/concerns/redis_cacheable_spec.rb:96:in `block (4 levels) in <top (required)>'