Bump piqi-erlang to upstream version

This is a merge of upstream to our fork.

Moreover, there's a crucial fix ensuring backwards and forwards compatibility:
https://github.com/yfyf/piqi-erlang/commit/2bd569302d75f8dc44b5e843f61fd190563369ec

I have tested the following combinations of `piqi` (binary) and `piqi-erlang` to work correctly with the user interface:

    piqi     piqi-erlang
    0.6.4    0.6.0-spil5
    0.6.4    $THIS
    $HEAD    0.6.0-spil5
    $HEAD    $THIS

where `$THIS` is this PR and `$HEAD` is the current head of piqi: https://github.com/spilgames/piqi/commit/2aee6bdd81570c423aa04eb6918270d01d845bc7

What this means is that it is safe to mix and `piqi` and `piqi-erlang` versions and release them in different orders and nothing will break.