Incompatibility with Sidekiq >= 6.1.0

[anero]

Due to the refactoring in the Fetch API made in sidekiq/sidekiq#4602 the gem is no longer compatible with latest Sidekiq versions.

[anero]

Submitted a new PR with a fix that also works with Sidekiq 6.5.0: #42

[rnubel]

Closing b/c #42 was merged. Thanks again!