BREAKING CHANGE: Move Joi to peerDependencies

[koooge]

Upgrade Joi v13 to v17.

fix #568

[vlucas]

Upgrading Joi is fine internally, but I am more worried about externally, i.e. people using the exported Joi object provided by Frisby for convenince in their own tests. Were there any braking changes from Joi 13 to 17? The version alone implies there likely are. I don't want people to upgrade Frisby and then wind up with tons of broken tests just because Joi change something.

[koooge]

Yes, Joi v17 has major changes absolutely. I was also wondering what to do about that. We can't upgrade Joi without breaking changes. Otherwise, frisby should not export Joi.
How about moving Joi to peerDependencies? It is also breaking changes tho.

const frisby = require('frisby');
// const Joi = frisby.Joi;
const Joi = require('joi');

[vlucas]

This is what SEMVer is for. We need to:

1. Make the change
2. Stop exporting Joi and make it a peerDependency (as you suggested) to prevent this problem in the future
3. Bump frisby up to a new major version (3.x) since it will be a potentially breaking change for those using the exported Joi object

[koooge]

joi in devDepencencies is needed for npm@6 and lower.

[koooge]

@vlucas I got it. Anyway updated this PR.

[vlucas]

Thanks @koooge