fix: Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-RACK-1061917
- https://snyk.io/vuln/SNYK-RUBY-RACK-3237233
- https://snyk.io/vuln/SNYK-RUBY-RACK-3237237
- https://snyk.io/vuln/SNYK-RUBY-RACK-3237240
- https://snyk.io/vuln/SNYK-RUBY-RACK-3356639
- https://snyk.io/vuln/SNYK-RUBY-RACK-3360233

Gemfile

@@ -118,7 +118,7 @@ group :test do
   gem 'faker', '~> 2.23'
   gem 'microformats', '~> 4.4'
   gem 'rails-controller-testing', '~> 1.0'
-  gem 'rspec-sidekiq', '~> 3.1'
+  gem 'rspec-sidekiq', '~> 4.0', '>= 4.0.0'
   gem 'simplecov', '~> 0.21', require: false
   gem 'webmock', '~> 3.18'
   gem 'rspec_junit_formatter', '~> 0.6'