Upgrade minimum ruby version to 2.4

Author: toduq

.rubocop.yml

@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.1
+  TargetRubyVersion: 2.4
   Exclude:
     - 'examples/**/*'
 
@@ -32,6 +32,8 @@ Style/Semicolon:
   Enabled: false
 Style/EachWithObject:
   Enabled: false
+Style/FrozenStringLiteralComment:
+  Enabled: false
 
 Naming/HeredocDelimiterNaming:
   Enabled: false

lib/line/bot/httpclient.rb

@@ -38,10 +38,8 @@ def http(uri)
           http.use_ssl = true
         end
 
-        if http_options
-          http_options.each do |key, value|
-            http.send("#{key}=", value)
-          end
+        http_options&.each do |key, value|
+          http.send("#{key}=", value)
         end
 
         http

line-bot-api.gemspec

@@ -17,7 +17,7 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]
 
-  spec.required_ruby_version     = '>= 2.0.0'
+  spec.required_ruby_version     = '>= 2.4.0'
 
   spec.add_development_dependency "addressable", "~> 2.3"
   spec.add_development_dependency "bundler", "~> 1.11" if RUBY_VERSION < "2.3"