use npm shrinkwrap

[jfirebaugh]

https://docs.npmjs.com/cli/shrinkwrap

We generate prebuilt bundles that include all dependencies, and ideally these bundles include the dependencies at versions that we have been commonly testing with. We want to avoid the scenario where a dependency happens to introduce a breaking change just as a new version of mapbox-gl-js is going to press, and it gets picked up by the automatic build.

This would also be an alternative fix for #1600. I'm assuming that if you are using shrinkwrap, npm install will update any outdated dependencies to the versions specified in npm-shrinkwrap.json.