0.4.0

• Change the upgrade process to modify the Gemfile.lock in place, with the
  option to "undo" (i.e. restore from backup) if the changes are not desired.
  This makes bundleup much faster since it doesn't have to run bundle update
  twice.
• Fixed a bug where color was mistakenly based on the newest version of a gem,
  even if a restriction prevented upgrading to that version.