def initialize(cache, stack = nil)
    @cache       = cache
    @stack       = stack
    @bundler_dir = Pathname.new("vendor/bundle")
    @stack_dir   = @stack ? Pathname.new(@stack) + @bundler_dir : @bundler_dir
  end
@param [LanguagePack::Cache] cache object
@param [String] stack buildpack is running on