Uses https://github.com/uber/npm-shrinkwrap instead of the built-in "npm shrinkwrap" since it works more reliably.
Specifically, "npm install" doesn't fail depending on node_modules/ being installed in the local cache or not.
It also makes npm-shrinkwrap.json easier to diff by more consistently ordering its output between runs.
If you need any convincing that this is a problem, look at the over 400 issues related to "shrinkwrap"
in https://github.com/npm/npm/search?q=shrinkwrap&type=Issues&utf8=%E2%9C%93
