If bundler had a `fetch` command

fetchgems.rb

require 'bundler'
require 'rubygems'
require 'rubygems/gem_runner'
require 'fileutils'

without = [:development, :test]

runner = Gem::GemRunner.new
definition = Bundler::Definition.build('Gemfile', 'Gemfile.lock', nil)
resolver = definition.resolve

lazy_specs = resolver.for(definition.dependencies.reject {|d| (d.groups - without).empty?}, [], false, true).to_a.uniq

FileUtils.mkdir_p 'vendor/cache'
FileUtils.cd 'vendor/cache'

lazy_specs.each do |spec|
  runner.run ['fetch', spec.name, '-v', spec.version.to_s]
end