please update depencencies avoid error on build

I try to use imagemin and imagemin-webp on wepback project.
And then still trying to do something good, i get error about dependency exec-buffer. It's give me an error:

``` ERROR Failed to compile with 3 errors 10:44:30
This dependency was not found:

* child_process in ./node_modules/exec-buffer/node_modules/cross-spawn/index.js, ./node_modules/exec-buffer/node_modules/execa/index.js
and 1 other
```

Try to manage it my self but at the end it seems to be a dependencies error of depends packages of imagemin....

node v12.16.1
npm v6.14.4
yarn v1.22.4
windows 10

Start by reproducing the webpack build with the reported Node, npm, Yarn, and Windows versions. Inspect the dependency tree around exec-buffer, cross-spawn, and execa, then verify that the build no longer reports the missing child_process dependency.

索引モデルが issue の本文から書いたものです。