Update sidekiq dependency in gemspec

Update sidekiq dependency in gemspec to accomodate versions 2.13.1 & greater. Present version of sidekiq is [2.13.1](https://github.com/mperham/sidekiq/tree/v2.13.1), with head at [2.14.0](https://github.com/mperham/sidekiq/tree/2b5feb981df9150f26b8aac2451374816f7d0fbf).