fix hitbtc constructor name in cctx (#2156)

the name of the constructor is hitbtc, not hitbtc2, as visible in https://github.com/ccxt/ccxt/blob/master/ccxt.js#L118
the current wrong name results in this error when using hitbtc in zenbot
TypeError: ccxt.hitbtc2 is not a constructor
    at publicClient (/home/me/workspaces/crypto/zenbot/extensions/exchanges/hitbtc/exchange.js:12:41)

extensions/exchanges/hitbtc/exchange.js

@@ -9,7 +9,7 @@ module.exports = function container (conf) {
   var public_client, authed_client
 
   function publicClient () {
-    if (!public_client) public_client = new ccxt.hitbtc2({ 'apiKey': '', 'secret': '' })
+    if (!public_client) public_client = new ccxt.hitbtc({ 'apiKey': '', 'secret': '' })
     return public_client
   }
 
@@ -18,7 +18,7 @@ module.exports = function container (conf) {
       if (!conf.hitbtc || !conf.hitbtc.key || !conf.hitbtc.key === 'YOUR-API-KEY') {
         throw new Error('please configure your HitBTC credentials in ' + path.resolve(__dirname, 'conf.js'))
       }
-      authed_client = new ccxt.hitbtc2({ 'apiKey': conf.hitbtc.key, 'secret': conf.hitbtc.secret })
+      authed_client = new ccxt.hitbtc({ 'apiKey': conf.hitbtc.key, 'secret': conf.hitbtc.secret })
     }
     return authed_client
   }