mesos: fix build by forcing protobuf 3.6
mesos: fix build by forcing protobuf 3.6

mesos really seems to want protobuf 3.3, as dictated by maven. 3.6 does
seem to work though.

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs