update acorn

[goto-bus-stop]

to the same version as used by detective.

[pieterbeulque]

@substack — Can this get merged?

[paulirish]

In case this helps others: I'm using yarn and this resolutions block fixes things:

"resolutions": {
  "browserify/insert-module-globals/lexical-scope/astw": "github:goto-bus-stop/astw#new-acorn"
},