Support eslint v4

[alexander-akait]

No description provided.

[beefancohen]

Doesn't seem like anything needs to be done on our end, except upgrade peerDependencies. notice anything in the migration guide/changelog that would affect this plugin?

[ljharb]

@evcohen we should make sure that our peer dep is ^3 || ^4 tho, so it can be compatible with both.

Generally the changes are "invalid configs throw now", and some rules got stricter - so I don't think it would impact us.

[alexander-akait]

@ljharb we can use >=3 as in prettier/eslint-plugin-prettier#30

[ljharb]

That's dangerous, because then a v5 breaking change might break us. Just like you should never do *, you shouldn't ever do >.