Update the `Gemfile.lock` when required:

[Edouard-chin]

• At Shopify, when a dependency upgrade is almost complete, we enable the next version of the gem by default on all developers environment

  Our Gemfile then looks something like that

  unless ENV['DEPENDENCIES_PREVIOUS']
  enable_dual_booting
  gem 'rails', '~> 5.2.0'
  else
  gem 'rails', '~> 5.1.0'
  end

  This basically translate as "Boot with Rails 5.2.0 unless someone puts the DEPENDENCIES_PREVIOUS ENV".

  Because by default, the dualboot is enabled, when running bundle update <some_gem>, the Gemfile_next.lock will get updated, not the Gemfile.lock.

  This PR updates one lock or the other depending on which one was by default updated. However, if someone run DEPENDENCIES_NEXT=1 bundle update rails, the other lockfile will not be updated, that's because most likely if someone updates the lock witch a bootboot env var set, it means he solely wants to update that lock.