Upgrade nan to 2.10+ for node 10.

[jeffhung]

Upgrade nan to 2.10+ so node-geoip2 could build in node 10.

Otherwise, will encounter the following error:

  CXX(target) Release/obj.target/unix_dgram/src/unix_dgram.o
In file included from ../src/unix_dgram.cc:5:
In file included from ../../nan/nan.h:190:
../../nan/nan_maybe_43_inl.h:88:15: error: no member named 'ForceSet' in 'v8::Object'
  return obj->ForceSet(GetCurrentContext(), key, value, attribs);
         ~~~  ^

See: nodejs/nan#763