sample.rb

class GitHubPages
  VERSION = 10

  DEPENDENCIES = Dependency::Hell.new({
    "jekyll"               => "1.5.1",
    "kramdown"             => "1.3.1",
    "liquid"               => "2.5.5",
    "maruku"               => "0.7.0",
    "rdiscount"            => "2.1.7",
    "redcarpet"            => "2.3.0",
    "RedCloth"             => "4.2.9",
    "jemoji"               => "0.1.0",
    "jekyll-mentions"      => "0.0.9",
    "jekyll-redirect-from" => "0.3.1",
    "jekyll-sitemap"       => "0.3.0"
  })

  def self.versions
    DEPENDENCIES.to_h.merge({
      "github-pages" => VERSION.to_s,
      "ruby"         => RUBY_VERSION
    })
  end
end

class GitHubPages::Dependency
  attr_reader :gem_name, :default_version

  def initialize(gem_name, default_version)
    @gem_name, @default_version = gem_name, default_version
  end

  def version
    ENV.fetch env_name, default_version
  end

  def env_name
    @env_name ||= "PAGES_#{gem_name.upcase.gsub('-', '_')}"
  end

  def to_a
    [gem_name, version]
  end
end

class GitHubPages::Dependency::Hell
  def initialize(hash = {})
    @dependencies = hash.map { |name, version| Dependency.new name, version }
  end

  def to_h
    Hash[@dependencies.map { |o| o.to_a }]
  end
end