Fix Testing Instructions

I ran into a known issue[1] when trying to bring this gem into my project.

It turns out the instructions were just out of date. If you merely include the RSpec
file it misses other necessary requires. The noted file does everything you need
regardless of testing framework.

1. mhenrixon#741

README.md

@@ -582,8 +582,6 @@ end
 #spec/workers/bad_worker_spec.rb
 
 require "sidekiq_unique_jobs/testing"
-#OR
-require "sidekiq_unique_jobs/rspec/matchers"
 
 RSpec.describe BadWorker do
   specify { expect(described_class).to have_valid_sidekiq_options }