Enable lockfile support in Greenkeeper

[rbardini]

Description

Since we started using npm ci as the preferred method for installing dependencies, Greenkeeper PRs are failing the build. (see example)

This happens because Greenkeeper only updates package.json, not package-lock.json. If dependencies in the package lock do not match those in package.json, npm ci will exit with an error, instead of updating the package lock.

Expected results

Enable lockfile support in Greenkeeper, so Greenkeeper PRs do not fail the build anymore.