Bump Ruby to 2.7.4

Ruby 2.5 is EOL and Debian Stable ships 2.7.4

Author: Sjors

.cirrus.yml

@@ -1,11 +1,11 @@
 container:
-  image: ruby:2.5.5
+  image: ruby:2.7.4
 
 task:
   name: website
   env:
     JEKYLL_ENV: production
-    NOKOGIRI_USE_SYSTEM_LIBRARIES: true # speeds up installation of html-proofer 
+    NOKOGIRI_USE_SYSTEM_LIBRARIES: true # speeds up installation of html-proofer
     RUBYOPT: "-KU -E utf-8:utf-8"
 
   bootstrap_script: gem install bundler

.ruby-version

@@ -1 +1 @@
-2.5.5
+2.7.4

Gemfile

@@ -4,7 +4,7 @@ source 'https://rubygems.org'
 ## If you update the version here, also update it in .ruby-version, .cirrus.yml
 ## and README.md. Then push your branch and make sure Cirrus supports that
 ## version.
-ruby '2.5.5'
+ruby '2.7.4'
 
 ## If you add a new Gem below, run `bundle install` to install it.
 group :development do

Gemfile.lock

@@ -258,4 +258,7 @@ DEPENDENCIES
   html-proofer
 
 RUBY VERSION
-   ruby 2.5.5p157
+   ruby 2.7.4p191
+
+BUNDLED WITH
+   2.2.33

README.md

@@ -41,7 +41,7 @@ permalink: /en/2016/01/01/short-title
 ## Building
 
 This website is based on [Jekyll](https://jekyllrb.com/).  To build
-locally, [install Ruby 2.5.5](https://gorails.com/setup) using system
+locally, [install Ruby 2.7.4](https://gorails.com/setup) using system
 packages, [rvm](https://rvm.io), [rbenv](https://github.com/rbenv/rbenv), or another method.
 Then clone this repository and change directory into it: