Update webmock requirement from ~> 3.16.0 to ~> 3.22.0

Updates the requirements on [webmock](https://github.com/bblimke/webmock) to permit the latest version.
- [Changelog](https://github.com/bblimke/webmock/blob/master/CHANGELOG.md)
- [Commits](bblimke/webmock@v3.16.0...v3.22.0)

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

Signed-off-by: dependabot[bot] <[email protected]>

mail-notify.gemspec

@@ -34,7 +34,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rspec-rails", "~> 5.1"
   spec.add_development_dependency "standard", "~> 1"
   spec.add_development_dependency "sqlite3", "~> 1.6.2"
-  spec.add_development_dependency "webmock", "~> 3.16.0"
+  spec.add_development_dependency "webmock", "~> 3.22.0"
   spec.add_development_dependency "rspec-mocks", "~> 3.12.6"
 
   spec.add_dependency "actionmailer", ">= 5.2.4.6"