Update getrandom to 0.2

Currently, we get this warning:

```
WARNING in ./pkg/snek_bg.js 343:14-53
Critical dependency: the request of a dependency is an expression
 @ ./pkg/snek.js
 @ ./src/index.js
```

This is caused by `getrandom`, which is an indirect dependency through `rand`. The warning is already fixed on `master`, but not yet released. Once it is (as 0.2) we should update somehow.