Electron 4 support

[qarkai]

What version of WebTorrent Desktop?
0.20.0 (from https://aur.archlinux.org/packages/webtorrent-desktop/)

What operating system and version?
Arch Linux

What did you do?
Upgraded to electron-4.1.1

What did you expect to happen?
webtorrent-desktop started

What actually happened?

A JavaScript error occurred in the main process
Uncaught Exception:
TypeError: app.makeSingleInstance is not a function
    at Object.<anonymous> (/usr/lib/webtorrent-desktop/build/main/index.js:41:20)
    at Object.<anonymous> (/usr/lib/webtorrent-desktop/build/main/index.js:228:3)
    at Module._compile (internal/modules/cjs/loader.js:693:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:704:10)
    at Module.load (internal/modules/cjs/loader.js:602:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:541:12)
    at Function.Module._load (internal/modules/cjs/loader.js:533:3)
    at Module.require (internal/modules/cjs/loader.js:640:17)
    at require (internal/modules/cjs/helpers.js:20:18)
    at Object.<anonymous> (/usr/lib/webtorrent-desktop/index.js:1:156)

[qarkai]

Switch to branch greenkeeper/electron-4.1.4 doesn't help.

[vitlav]

$ sed -i -e "s/shouldQuit = app.makeSingleInstance(\(.*\))/shouldQuit = !app.requestSingleInstanceLock()/" src/main/index.js

[feross]

This is being handled in this PR: #1590