Fix multiple concurrent calls to install_and_run/2 (#58)

[azizk]

Fixes #58

I'm not sure if a Task.Supervisor should be used. WIP... 😄

[josevalim]

You want the operation to be atomic, otherwise you have races. So you either need the worker to be a named GenServer or you need to spawn the task directly in a regular Supervisor and use a unique child spec ID to avoid duplicates. :)

[azizk]

Hope this is better? 😄

[josevalim]

Glorious!