Require Ruby v2.5+ and nokogiri v1.11 (#143)

* require Ruby 2.5+

* Require nokogiri 1.11

closes #142

* rubocop TargetRubyVersion: 2.5

Author: toshimaru

.rubocop.yml

@@ -1,5 +1,5 @@
 AllCops:
-  TargetRubyVersion: 2.4
+  TargetRubyVersion: 2.5
   NewCops: enable
   Exclude:
     - "*.gemspec"

jekyll-toc.gemspec

@@ -17,8 +17,8 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec)/})
   spec.require_paths = ['lib']
 
-  spec.required_ruby_version = '>= 2.4'
+  spec.required_ruby_version = '>= 2.5'
 
   spec.add_dependency 'jekyll', '>= 3.9'
-  spec.add_dependency 'nokogiri', '~> 1.10'
+  spec.add_dependency 'nokogiri', '~> 1.11'
 end