Update LoadBalancer.cs

Possible bug if the LoadBalancer lives to process more than `int.MaxValue` messages.
`Interlocked.Increment(ref roundRobinCounter)` would 'overflow' to `int.MinValue` (a negative number) and would cause the indexOfThisDestination be negative too, causing an IndexOutOfRangeException
Merge pull request #216 from rlarno/patch-1

Update LoadBalancer.cs