Yarn provides much faster package installation, better resilience to npm
registry connection issues and most importantly for us, a much better
lockfile format.

This commit:

1. Replaces the npm shrinkwrap with the yarn lockfile using the
   following steps:

   1. Checkout latest version of master

   2. Remove node_modules folder and re-run `npm install`

   3. Run `yarn import` to generate a lockfile

2. Modifies the Makefile, Jenkinsfile, package.json and .travis.yml
   scripts to run `yarn` instead of `npm`.