Update to recent version of lerna dependency

[marcdumais-work]

This repo uses lerna 4.x, which is old and has several know security-related issues. It would be desirable to upgrade to a recent version. Alternatively, it seems lerna may not even be used, so removing it as a dependency would also work.

For reference, here's the lerna version used in upstream reposositories:

Theia: 7.1.1

Blueprint: 6.6.2