Relax bundler version restriction

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

fluent-plugin-fluent-package-update-notifier.gemspec

@@ -26,7 +26,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_development_dependency "bundler", "~> 2.6.2"
+  spec.add_development_dependency "bundler", "~> 2.4"
   spec.add_development_dependency "rake", "~> 13.2.1"
   spec.add_development_dependency "test-unit", "~> 3.6.7"
   spec.add_runtime_dependency "fluentd", [">= 0.14.10", "< 2"]