Some minor fixes: default wss endpoint, dependencies upgrade and futures general endpoint ping

[ytskuh]

1. The wss endpoint with 9443 port seems to have issue according to websocket receive MarketData error #123. By simply adjusting the default endpoint the problem can be fixed.
2. The mock module has deprecated since mockito 1.0.0. Upgraded the dependencies and affected test module. Some other dependencies are also safely upgraded.
3. FuturesGeneral::ping() doesn't work and since there lacks a test for that, this issue has not been discovered. Added a test module for that and fixed the function.

I'm a Rust beginner and tried my best to make this little contribution.