Bug#1023535: transition: protobuf

[Christoph Berg <myon@debian.org>]

Re: László Böszörményi
> On the other hand I'm done with the rebuilds and patched all issues,
> this transition may start immediately at your decision. The only
> downside is that the Sid version of cura-engine is FTBFS and to fix
> it, the libarcus transition (only affecting this package) will need to
> be done.

I'll take care of cura and deps when the transition starts.
(Unfortunately the newest libarcus package doesn't work with the old
protobuf, so we can't upload that yet.)

Christoph