Add support ActiveSupport::Testing::TimeHelpers

[billthompson]

ActiveSupport::Testing::TimeHelpers stubs Time.now automatically, so Timecop
won't be a required dependency when testing Stoplight inside of a gem
consumer's project.

---

Thanks for making this gem! If you have any specific contributor guidelines that I haven't followed, just let me know.

[tfausak]

Thanks for opening this pull request! I'm all about keeping dependencies in check. However Stoplight doesn't depend on Active Support — not even for development. Timecop itself has no dependencies, so I'd prefer to keep using it instead of Active Support.

[billthompson]

Thanks for the quick response!

I'm sorry I wasn't more clear in my description. This PR doesn't change Stoplight's dependencies or existing development flow. Notice that the specs weren't updated and all tests continue to pass.

That's because Time.now is the same as Time.new when called with no arguments. By accepting this change, consumers who use ActiveSupport::Test::TimeHelpers instead of Timecop can successfully time travel when testing.

I hope this is more clear, but please let me know if you have any questions.

[tfausak]

Oh, I see. This is for other test suites that happen to use Stoplight in them. Makes total sense.

Thanks for the patch, and I'll be sure to use Time.now in the future!