Update all dependencies

[lemonmade]

This PR updates all of our linting dependencies to their most recent versions and adds the new rules they introduced. Most notably, the following rules now apply:

• Line comments should be above the line they are commenting, not at the end of it (definitely open to discussion, wasn't sure about this one).
• Always include a description when using the Symbol function.
• A bunch of spacing-related rules for Flow.
• In Flow, prevent the usage of Function as a type (I chose not to restrict Object and any since I find them to be useful occasionally, not sure about it though).
• Prevent the usage of done and returning a promise together in mocha tests.
• Require whitespace around directives, like 'expose Foo'.

cc/ @Shopify/javascript @bouk thoughts?

[arapehl]

The line comment one is a bit tricky 'cause sometimes you don't want to break up a clean block of assignments but want a small comment next to one item.

Otherwise, these all seem reasonable to me.

[bouk]

maybe call it 'export' instead of 'expose'

[lemonmade]

reping @Shopify/javascript. I think I am still leaning towards keeping the recommendation for line comments to appear above the line they are documenting, since I think that's usually the better choice and it's worth the consistency. Added a few more updates since I initially did this PR, so a few more things that might need discussion:

• When using Node modules, prefer module.exports.foo over exports.foo (not sure about this one)
• Only one top-level suite per file for Mocha tests
• In React components, no explicit children prop (i.e., prefer <Foo>{bar}</Foo> over <Foo children={bar} />
• @flow comments should be done as line comments

[bouk]

I don't think preferring module.exports or export matters because we recommend using ES6 module syntax

I don't think we should have a preference of what flow comment style to use

[mikkoh]

I think it's still worth having the rule for module.exports.foo. As @bouk said it's an edge case now but in my experience exports.foo may cause issues with tooling. Just safer to use module.exports.foo.

[lemonmade]

Keeping the module.exports rule, removed the preference on the flow comment, and removed no-sync (which was disabled in pretty much all node projects I've seen).

[bouk]

lgtm