Update rubocop requirement from ~> 0.40.0 to ~> 0.81.0

Updates the requirements on [rubocop](https://github.com/rubocop/rubocop) to permit the latest version.
- [Release notes](https://github.com/rubocop/rubocop/releases)
- [Changelog](https://github.com/rubocop/rubocop/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop@v0.40.0...v0.81.0)

---
updated-dependencies:
- dependency-name: rubocop
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

resque-scheduler.gemspec

@@ -54,7 +54,7 @@ Gem::Specification.new do |spec|
 
   # We pin rubocop because new cops have a tendency to result in false-y
   # positives for new contributors, which is not a nice experience.
-  spec.add_development_dependency 'rubocop', '~> 0.40.0'
+  spec.add_development_dependency 'rubocop', '~> 0.81.0'
 
   spec.add_runtime_dependency 'mono_logger', '~> 1.0'
   spec.add_runtime_dependency 'redis', '>= 3.3'