fix: made lerna a peerDependency to avoid installing it multiple times

[sebinsua]

Description

lerna@^2.9.0 should be a peerDependency (and for the purposes of the tests a devDependency) instead of a dependency.

Motivation and Context

This will avoid situations in which multiple versions of lerna get installed into a monorepo and it barfs when you try to run one of the commands (with "Incompatible local version of lerna detected!") due to your lerna.json#version not matching the version which has decided to run. (This sometimes happens if yarn has decided to install your packages into a slightly weird folder structure and the version of lerna which is being picked up is not the version that you installed within your root package.json.)

Usage examples

N/A

How Has This Been Tested?

I ran yarn test at the root of the monorepo.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[Sergiioo]

I've just hit this issue where I get the same incompatible lerna version warning after running yarn add @storybook/addon-knobs in a lerna monorepo project, so this PR would be really useful.

[marionebl]

Hey thanks for contributing - I think this is superseeded by #406.