Update webmock requirement from ~> 3.18.1 to ~> 3.19.1

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@v3.18.1...v3.19.1)

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

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

wpscan.gemspec

@@ -32,5 +32,5 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'simplecov',           '~> 0.21.0'
   s.add_development_dependency 'simplecov-lcov',      '~> 0.8.0'
   s.add_development_dependency 'stackprof',           '~> 0.2.12'
-  s.add_development_dependency 'webmock',             '~> 3.18.1'
+  s.add_development_dependency 'webmock',             '~> 3.19.1'
 end