fix: Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-ACTIVESUPPORT-3237242
- https://snyk.io/vuln/SNYK-RUBY-ACTIVESUPPORT-3360028
- https://snyk.io/vuln/SNYK-RUBY-ACTIVESUPPORT-569598
- https://snyk.io/vuln/SNYK-RUBY-ADDRESSABLE-1316242
- https://snyk.io/vuln/SNYK-RUBY-ASCIIDOCTOR-72630
- https://snyk.io/vuln/SNYK-RUBY-KRAMDOWN-1087436
- https://snyk.io/vuln/SNYK-RUBY-KRAMDOWN-585939
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-1055008
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-1293239
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-1726792
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-2413994
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-2620374
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-2630623
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-2630898
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-2840634
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-3052880
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-3357692
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-3357693
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-459107
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-534637
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-552159
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-72433
- https://snyk.io/vuln/SNYK-RUBY-RAKE-552000
- https://snyk.io/vuln/SNYK-RUBY-TZINFO-2958048

Gemfile

@@ -2,26 +2,26 @@ source 'https://rubygems.org'
 ruby '2.4.2'
 
 # essential
-gem 'nanoc', '~> 4.7'
-gem 'nanoc-conref-fs', '~> 0.7'
-gem 'nanoc-redirector', '~> 0.2'
+gem 'nanoc', '~> 4.9', '>= 4.9.4'
+gem 'nanoc-conref-fs', '~> 0.7', '>= 0.7.0'
+gem 'nanoc-redirector', '~> 0.3', '>= 0.3.0'
 
 # rendering
-gem 'nanoc-html-pipeline', '~> 0.3'
+gem 'nanoc-html-pipeline', '~> 0.3', '>= 0.3.5'
 gem 'gemoji', '2.1.0'
-gem 'html-pipeline-rouge_filter', '~> 1.0'
-gem 'extended-markdown-filter', '~> 0.4'
-gem 'html-pipeline-asciidoc_filter', '~> 1.5'
+gem 'html-pipeline-rouge_filter', '~> 1.0', '>= 1.0.7'
+gem 'extended-markdown-filter', '~> 0.6', '>= 0.6.0'
+gem 'html-pipeline-asciidoc_filter', '~> 1.5', '>= 1.5.3'
 gem 'github-markdown', '~> 0.6.9'
-gem 'kramdown'
+gem 'kramdown', '>= 2.3.1'
 gem 'kramdown-parser-gfm'
 
-gem 'activesupport', '~> 4.2'
-gem 'rake'
+gem 'activesupport', '~> 6.1', '>= 6.1.7.3'
+gem 'rake', '>= 12.3.3'
 gem 'pry'
 gem 'pry-coolline'
 
 group :test do
   gem 'colorize', '~> 0.8.1'
-  gem 'html-proofer', '~> 3.7'
+  gem 'html-proofer', '~> 3.12', '>= 3.12.0'
 end