chore(deps-dev): update webmock requirement from ~> 3.17.0 to ~> 3.18.1 (#408)

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](bblimke/webmock@v3.17.0...v3.18.1)

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

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

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

appium_lib_core.gemspec

@@ -29,7 +29,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'yard', '~> 0.9.11'
   spec.add_development_dependency 'minitest', '~> 5.0'
   spec.add_development_dependency 'minitest-reporters', '~> 1.1'
-  spec.add_development_dependency 'webmock', '~> 3.17.0'
+  spec.add_development_dependency 'webmock', '~> 3.18.1'
   spec.add_development_dependency 'rubocop', '1.35.0'
   spec.add_development_dependency 'appium_thor', '~> 1.0'
   spec.add_development_dependency 'parallel_tests'