Fix error on Windows caused by setBadge

[Goldob]

On each startup with npm start on Windows, the following error appears in the terminal:

TypeError: app.setBadgeCount is not a function
    at Object.setBadge (C:\Users\Goldob\Projekty\NodeJS\webtorrent-desktop\build\main\dock.js:37:7)
    at EventEmitter.ipc.on (C:\Users\Goldob\Projekty\NodeJS\webtorrent-desktop\build\main\ipc.js:52:43)
    at emitTwo (events.js:106:13)
    at EventEmitter.emit (events.js:191:7)
    at EventEmitter.ipc.emit (C:\Users\Goldob\Projekty\NodeJS\webtorrent-desktop\build\main\ipc.js:150:13)
    at EventEmitter.<anonymous> (C:\Users\Goldob\Projekty\NodeJS\webtorrent-desktop\node_modules\electron-prebuilt\dist\resources\electron.asar\browser\api\web-contents.js:156:13)
    at emitTwo (events.js:106:13)
    at EventEmitter.emit (events.js:191:7)

This PR fixes that by ensuring that we are running either Mac or Linux with Unity before calling the API.

[mathiasvr]

LGTM