Add voxel-engine dependency, module passed to voxel-server + voxel-client

Simplifies dependency management (ensures same voxel-engine is used
for both client/server, instead of having each voxel-server/voxel-client
module specify the version, which may be a custom version). Requires:

deathcap/voxel-server@f204ec9c8a90ba82e89afb733b76622f00d83047 Accept voxel-engine module as required 'engine' option, remove dependency
deathcap/voxel-client@06807aa1e808968f0c3f45f33806362c4327a546 Accept voxel-engine module as required 'engine' option, remove dependency

Commit message generated by https://github.com/deathcap/lmno-cl