Update webmock requirement to ~> 3.4.0

Updates the requirements on [webmock](https://github.com/bblimke/webmock) to permit the latest version.
- [Release notes](https://github.com/bblimke/webmock/releases)
- [Changelog](https://github.com/bblimke/webmock/blob/master/CHANGELOG.md)
- [Commits](https://github.com/bblimke/webmock/commits/v3.4.0)

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

dependabot-core.gemspec

@@ -36,5 +36,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "rspec", "~> 3.7.0"
   spec.add_development_dependency "rspec-its", "~> 1.2.0"
   spec.add_development_dependency "rubocop", "~> 0.55.0"
-  spec.add_development_dependency "webmock", "~> 3.3.0"
+  spec.add_development_dependency "webmock", "~> 3.4.0"
 end