There were a couple of issues with the initial Yarn lockfile created via
`yarn import`:

1. `yarn check` reported several inconsistencies.

2. `phantomjs-prebuilt` needed to be upgraded to resolve a compatibility
   issue with Yarn.

This commit resolves these issues by just removing and recreating the
Yarn lockfile. In the process all dependencies have been upgraded to the
latest versions compatible with the constraints in package.json.