Fix compile script to work on Windows.

[MicahZoltu]

https://github.com/Uniswap/uniswap-v2-core/blob/d2ddbd95e8500cfd3669ab81b64f48956521ea6b/package.json#L22

rm is not a valid command on Windows. Consider using an NPM module for this instead, or an inline node script with node -e '...'. If you bump the min version of Node supported to v12 (LTS) then you can replace the script line with:

node -e \"require('fs').rmdir('./build/', { recursive: true }, error => { if (error !== null) console.error(error) })\" && waffle waffle.json

(could be simplified if you drop the error logging)

[NoahZinsmeister]

fixed! uses rimraf now