quitAndInstall and downloadUpdate don't work properly on LInux

I’ve faced a few issues while using it on Linux:

1. quitAndInstall doesn’t work properly. It spawns the child process and the runs app.quit(), which doesn’t quit until the child process will quit. Here is more info on that: https://github.com/electron/electron/issues/2208. In my own implementation I’ve wrote my own quitAndInstall which, instead of spawning child process, uses app.relaunch() (taken from the last comment of this issue).
2. when downloading the updates file and replacing the original one with it, it gets corrupted (when I run it I get the error, though if I just download the AppImage file and run it directly, it will work). I’ve fixed it in the following way: downloaded the update file into different directory, then removed the old file, then use app.relaunch with execPath specified, so it’d run the new file instead of old one.

Can you look into that? Would be lovely to have it integrated in electron-simple-updater instead of writing some workarounds for downloadUpdate and quitAndInstall. I can also make a PR on that, if you want.

[megahertz]

@serge1peshcoff

Nice work, thanks! I’m going to apply some of your solutions.

[megahertz]

In my tests v1.4.2 works correctly. Feel free to reopen if you still have some errors.