postinstall fails when installing new beta builds

https://github.com/xtermjs/xterm.js/commit/6dae811dae20c7af85fa09cafd59c5ae774a58ed#r125024162

> bin folder is excluded from npm package, trying to install beta release fails as the script path does not exist