Fix syntax error in exchanges/kraken.js

[kacperzuk]

This fixes this syntax error:

gekko/exchanges/kraken.js:154
    if (err))
            ^
SyntaxError: Unexpected token )
    at Object.exports.runInThisContext (vm.js:53:16)
    at Module._compile (module.js:513:28)
    at Object.Module._extensions..js (module.js:550:10)
    at Module.load (module.js:458:32)
    at tryModuleLoad (module.js:417:12)
    at Function.Module._load (module.js:409:3)
    at Module.require (module.js:468:17)
    at require (internal/module.js:20:19)
    at checkExchangeTrades (gekko/plugins/tradingAdvisor.js:26:22)
    at Actor.prepareHistoricalData (gekko/plugins/tradingAdvisor.js:116:3)

It was introduced in commit 620f9aa .

[askmike]

nice catch!