nuspec dependency fixes
nuspec dependency fixes

Explicitly don't add any dependencies for "old" platforms, to avoid unnecessary
dependencies in those cases.

Fixes issue #1203.

google/protobuf,google/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,google/protobuf,google/protobuf,Livefyre/protobuf,Livefyre/protobuf,google/protobuf