Loosen peers to allow for v3 of eslint

[samouri]

See b6a59ff

can we make it compatible with both v3 and v4 instead? wp-calypso is currently blocked from using v4 do to: Automattic/wp-calypso#18106

[aduth]

While this should be fine in its current form, I don't want to maintain support for eslint-plugin-wpcalypso in perpetuity since we may introduce rules into this config that depend on a newer version of the plugin. Since that's not the case and 4.x was a peer dependency upgrade on ESLint, there should not be an issue to use this as a temporary resolution until babel-eslint improves its support for ESLint 4.x.