MAX_ARRAY_INDEX = 2 ** 53 - 1; Unexpected token *

Hi Jonathan,

we use one of your node page and we seem to have an issue with your latest release change (from yesterday):

```
10:29:58 [exec] /node_modules/@jonkemp/package-utils/index.js:6
10:29:58 [exec] const MAX_ARRAY_INDEX = 2 ** 53 - 1;
10:29:58 [exec] ^
10:29:58 [exec]
10:29:58 [exec] SyntaxError: Unexpected token *
10:29:58 [exec] at createScript (vm.js:56:10)
10:29:58 [exec] at Object.runInThisContext (vm.js:97:10)
10:29:58 [exec] at Module._compile (module.js:549:28)
10:29:58 [exec] at Object.Module._extensions..js (module.js:586:10)
10:29:58 [exec] at Module.load (module.js:494:32)
10:29:58 [exec] at tryModuleLoad (module.js:453:12)
10:29:58 [exec] at Function.Module._load (module.js:445:3)
10:29:58 [exec] at Module.require (module.js:504:17)
10:29:58 [exec] at require (internal/module.js:20:19)
10:29:58 [exec] at Object. (/node_modules/flat-util/index.js:1:100)
10:29:58
```
Anything you can do from your side? Or hint for us to resolve this? Thanks!

Btw, we are currently using node with version v6.13.1

Cheers,

--hai