failed buy orders on bittrex due to insufficient funds

[Exalovic]

I've been getting several of these errors after the commits on dec 30:

It's very frustrating because it means I have to refund the coins again, leading to losses

[cryptoeax]

oops, it seems like the code has never taken the amount of funds available at the buy side into account!!!

https://github.com/cryptoeax/arbbot/blob/6318feb039122a6fc02ccaa824071811bea4b626/bot/Arbitrator.php#L281

[cryptoeax]

This fix was reverted in 9853a22aa15140986b6a106be8b4bf1c1776ff5c.

[cryptoeax]

I pushed a commit that I believe should have resolved a possible root cause of this.