Fix async_pi_estimate deprecation

[amir-s]

👋
Since v8::Local<v8::Value> Call(int argc, v8::Local<v8::Value> argv[]); is now deprecated (#733), I'm updating the example to use the new API.

before:

> node-gyp rebuild

  CXX(target) Release/obj.target/addon/addon.o
  CXX(target) Release/obj.target/addon/pi_est.o
  CXX(target) Release/obj.target/addon/sync.o
  CXX(target) Release/obj.target/addon/async.o
../async.cc:50:15: warning: 'Call' is deprecated [-Wdeprecated-declarations]
    callback->Call(2, argv);
              ^
../node_modules/nan/nan.h:1567:3: note: 'Call' has been explicitly marked deprecated here
  NAN_DEPRECATED inline v8::Local<v8::Value>
  ^
../node_modules/nan/nan.h:98:40: note: expanded from macro 'NAN_DEPRECATED'
# define NAN_DEPRECATED __attribute__((deprecated))
                                       ^
1 warning generated.
  SOLINK_MODULE(target) Release/addon.node

after:

> node-gyp rebuild

  CXX(target) Release/obj.target/addon/addon.o
  CXX(target) Release/obj.target/addon/pi_est.o
  CXX(target) Release/obj.target/addon/sync.o
  CXX(target) Release/obj.target/addon/async.o
  SOLINK_MODULE(target) Release/addon.node

[kkoopa]

Good catch. Thank you.