v1.2.0 - Support Sidekiq 7.1 and update counts matching

POTENTIAL BREAKING CHANGE:

• Remove default .once when using singular matcher [#2]
  It could break ambiguous negative assertions. Read more in the PR description

Improvements:

• Add support to Sidekiq-Pro 7.1 changes

• Add chainable matcher to verify the number of enqueued jobs against a max/min number:

  it { expect {...}.to enqueue_sidekiq_job(SampleJob).more_than(5).times }
  it { expect {...}.to enqueue_sidekiq_job(SampleJob).less_than(5).times }
  it { expect {...}.to enqueue_sidekiq_job(SampleJob).at_least(5).times }
  it { expect {...}.to enqueue_sidekiq_job(SampleJob).at_most(5).times }