diff --git a/Gemfile b/Gemfile
index 5714bad..fd6fc63 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,4 +1,12 @@
-source "https://rubygems.org"
-gem "github-pages", group: :jekyll_plugins
-gem "kramdown-parser-gfm"
-gem "mdl"
+# frozen_string_literal: true
+
+source 'https://rubygems.org'
+
+git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
+
+# Synchronize with https://pages.github.com/versions
+ruby '>=2.7.1'
+
+gem 'github-pages', group: :jekyll_plugins
+gem 'kramdown-parser-gfm'
+gem 'mdl'