Don't log INFO messages to `stderr`

The way the `logger` is used in `sprockets` is violating `POSIX` standards:
Only errors should go to `stderr`, not "INFO" messages.

I've attached a simple fix which sets the level to "WARN".

Perhaps a more correct solution would be to instantiate two loggers (one for `stdout`, one for `stderr`) and sending messages to the output appropriate for the level.