fix(config-lerna): support lerna > 2.4

[marionebl]

• Picks up: https://github.com/marionebl/commitlint/pull/88
• Add test setup
• Assert lerna-related assumptions
• Test lerna 2.0 and lerna 2.4 support

[marionebl]

Release https://github.com/marionebl/commitlint/releases/tag/v4.1.1

[QuentinRoy]

As it seems this is relying on some unstable part of lerna, I was wondering if it might be a good idea to specify lerna's version (in the dependencies) as strict (i.e. without the ^, e.g. 2.4.x). Doing this would almost always result in a second version of lerna being installed, but would significantly reduce the risk for this package to suddenly fail on new versions of lerna.

[marionebl]

This indeed relies on currently undocumented internal lerna API. I plan on asking the lerna project to publish some of it, did not come around to do that yet.

I changed config-lerna to pick up the lerna version used by the project, so pinning our own devDependency (will have to move lerna to that for config-lerna) would not achieve better stability.

I'd keep it in its current state (optimised for guaranteed equivalence of pkg listings) for now.