rails_version_specifier

# File railties/lib/rails/generators/app_base.rb, line 254
      def rails_version_specifier(gem_version = Rails.gem_version)
        if gem_version.prerelease?
          next_series = gem_version
          next_series = next_series.bump while next_series.segments.size > 2

          [">= #{gem_version}", "< #{next_series}"]
        elsif gem_version.segments.size == 3
          "~> #{gem_version}"
        else
          patch = gem_version.segments[0, 3].join(".")
          ["~> #{patch}", ">= #{gem_version}"]
        end
      end