Update webmock requirement from 1.21.0 to 3.13.0

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](bblimke/webmock@v1.21.0...v3.13.0)

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

Gemfile

@@ -12,5 +12,5 @@ group :test do
   gem 'rb-readline'
   gem 'rspec', '~> 3.10.0'
   gem 'rubocop', '~> 1.6'
-  gem 'webmock', '1.21.0'
+  gem 'webmock', '3.13.0'
 end