v2.1.1 Release

Improvements

• The domain name wss://openapi-ws.bitmart.com will not provide Futures 1.0 Websocket services.
  Please use the domain name wss://openapi-ws-v2.bitmart.com to access Futures 2.0 Websocket services
• Rename CloudConst::WS_CONTRACT_PUBLIC_URL_PRO to CloudConst::WS_FUTURES_PUBLIC_URL_PRO
  and change the value from wss://openapi-ws.bitmart.com/api?protocol=1.1 to wss://openapi-ws-v2.bitmart.com/api?protocol=1.1
• Rename CloudConst::WS_CONTRACT_PRIVATE_URL_PRO to CloudConst::WS_FUTURES_PRIVATE_URL_PRO
  and change the value from wss://openapi-ws.bitmart.com/user?protocol=1.1 to wss://openapi-ws-v2.bitmart.com/user?protocol=1.1