Update webmock requirement from ~> 1.24 to ~> 3.8

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@v1.24.0...v3.8.3)

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

Author: dependabot-preview[bot]

line-bot-api.gemspec

@@ -23,5 +23,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler", "~> 1.11" if RUBY_VERSION < "2.3"
   spec.add_development_dependency 'rake', "~> 10.4"
   spec.add_development_dependency "rspec", "~> 3.0"
-  spec.add_development_dependency "webmock", "~> 1.24"
+  spec.add_development_dependency "webmock", "~> 3.8"
 end