Fix time comparing in have_enqueued_sidekiq_job

[paulslaby]

Following spec fails for me from time to time and this PR should fix it.

    subject.perform(args)
    expect(described_class).to have_enqueued_sidekiq_job(args).in(15.minutes)

[coveralls]

Coverage increased (+0.007%) to 97.534% when pulling 9cde5b01929259852abc1e8a8fd9daac0f820dd6 on paulslaby:develop into ccce13c71ebfe848bc35ec79fc62f63714f9e3e1 on philostler:develop.

[wspurgin]

I prefer the approach taken in #182 to line up with how Sidekiq calculates it. Same offer as over there. If you haven't moved on from Ruby/Sidekiq and rspec-sidekiq, feel free to rebase/restart and I'll review.

[wspurgin]

Closing in favor of #194