Update rubocop-discourse requirement from = 3.3.0 to = 3.4.0 (#38)

Updates the requirements on [rubocop-discourse](https://github.com/discourse/rubocop-discourse) to permit the latest version.
- [Commits](discourse/rubocop-discourse@v3.3.0...v3.4.0)

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

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mini_scheduler.gemspec

@@ -30,5 +30,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "guard-rspec", "~> 4.0"
   spec.add_development_dependency "redis", ">= 4.0"
   spec.add_development_dependency "rake", "~> 13.0"
-  spec.add_development_dependency "rubocop-discourse", "= 3.3.0"
+  spec.add_development_dependency "rubocop-discourse", "= 3.4.0"
 end