Ensure RSpec.configure is defined on loading rspec/sidekiq/matchers:

Loading "rspec/core" defines "RSpec.configure" when running, for example,
"rake db:test:prepare". Without loading "rspec/core", you get the
error: "undefined method `configure' for RSpec:Module"
Merge pull request #3 from centaure/master

Define  "RSpec.configure" method when running "rake db:test:prepare".