upgrade webmock

Author: Yoshiyuki Kinjo

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