src: fix compilation error on node 0.11.13

The compiler was complaining when building the module for node 0.11.13

../src/unix_dgram.cc:119:13: error: call to 'NanNew' is ambiguous
  argv[0] = NanNew(err);
            ^~~~~~
../node_modules/nan/nan.h:1249:36: note: candidate function
  NAN_INLINE v8::Local<v8::Number> NanNew(double val) {
                                   ^
../node_modules/nan/nan.h:1253:37: note: candidate function
  NAN_INLINE v8::Local<v8::Integer> NanNew(int val) {
                                    ^
../node_modules/nan/nan.h:1257:36: note: candidate function
  NAN_INLINE v8::Local<v8::Uint32> NanNew(unsigned int val) {
                                   ^
../node_modules/nan/nan.h:1261:37: note: candidate function
  NAN_INLINE v8::Local<v8::Boolean> NanNew(bool val) {
                                    ^