Need to update a Gemfile.lock but don't want to install the gems? No problem! Run this in the same directory as your Gemfile!

bundle-update-lockfile.rb

#!/usr/bin/env ruby

require 'bundler'

gems = ARGV

if gems.empty?
  puts "Updating all gems"

  Bundler.definition(true)
else
  puts "Updating gems: #{gems.join(', ')}"

  lock = Bundler.read_file(Bundler.default_lockfile)
  names = Bundler::LockfileParser.new(lock).specs.map{ |s| s.name }

  Bundler.definition(:gems => gems, :sources => [])
end

d = Bundler.definition

puts "Resolving remotely..."

d.resolve_remotely!
d.lock(Bundler.default_lockfile)

puts "Updated lockfile!"

if Bundler.root.join("vendor/cache").exist?
  puts "Downloading .gem's to update vendor/cache"

  d.specs.sort_by { |s| s.full_name}.each do |s|
    Bundler::Fetcher.fetch(s)
  end

  Bundler.load.cache
end

puts "Done! Enjoy!"