Add parens to single-parameter arrow functions

[GoodForOneFare]

Our linting config will carp about arrow-parens if they're omitted.

foo.then(bar => qux());

should become:

foo.then((bar) => qux());

[GoodForOneFare]

Will be fixed if this recast PR is accepted: benjamn/recast#288.

[lemonmade]

The PR to recast is good, but does the ESLint run not fix this?

[GoodForOneFare]

You'd think, but I don't see any fix text in https://github.com/eslint/eslint/blob/master/lib/rules/arrow-parens.js. A quick 🎩 with eslint -fix enabled added a newline, but didn't tweak function params.

[GoodForOneFare]

Fix has been merged into Recast. I'll update our config.