Missing `yarn` dependency?

`package.json` install script references `yarn`, but none of the dependencies install it, should it be added there? Running `npm i minidump` as instructed in the README generates the following error:
```
npm error command sh -c yarn submodule && node build.js
npm error sh: 1: yarn: not found
```

Running `npm i yarn` before trying to install minidump seems to help (`yarn@1.22.22` for `minidump@0.26.0` worked for me at the time of writing this report).

Happy to make a PR if it's as simple as adding it to package.json (but I admit I know nothing about yarn and if there's some reason you wouldn't want to do that as there often seems to be with overenthusiastic build systems ^_^)