Update gem dependency to Ruby 2.4.2+

Author: simukappu

README.md

@@ -50,7 +50,7 @@ Or just download specify your Ruby library path. Below is the sample for specify
     export RUBYLIB=$RUBYLIB:/path/to/aws-fluent-plugin-kinesis/lib
 
 ## Dependencies
- * Ruby 2.3.0+
+ * Ruby 2.4.2+
  * Fluentd 0.14.22+ (td-agent v3.1.0+)
 
 ## Basic Usage

fluent-plugin-kinesis.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |spec|
   spec.files         = `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
-  spec.required_ruby_version = '>= 2.3'
+  spec.required_ruby_version = '>= 2.4.2'
 
   spec.add_dependency "fluentd", ">= 0.14.22", "< 2"