Make Electron a devDependency
Make Electron a devDependency

I moved it from devDependencies to dependencies when we added the app
to npm. But now that that's gone, let's move it back.

Functionally, this causes no difference since electron-packager
automatically excludes `electron` and all devDependencies from the
packaged app.

feross/webtorrent-app,michaelgeorgeattard/webtorrent-desktop,feross/webtorrent-desktop,feross/webtorrent-desktop,webtorrent/webtorrent-desktop,feross/webtorrent-app,feross/webtorrent-app,feross/webtorrent-desktop,webtorrent/webtorrent-desktop,michaelgeorgeattard/webtorrent-desktop,michaelgeorgeattard/webtorrent-desktop,webtorrent/webtorrent-desktop