Update getrandom to 0.2

[LukasKalbertodt]

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.