Use go-loggly from go-slog

- related to https://github.com/GigaOM/legacy-pro/issues/3806, and specifically [this](https://github.com/GigaOM/go-loggly/pull/3#issuecomment-49441761) discussion.
- using `GO_Slog::log()` but replacing it with a `go-loggly` implementation.