Account for time drifting backwards on servers with the moved too quickly check changes.

src/main/java/net/minecraft/server/NetServerHandler.java

@@ -280,6 +280,9 @@ public class NetServerHandler extends NetHandler implements ICommandListener {
 
             // CraftBukkit start - make the movement speed check behave properly under tick degradation.
             int elapsedTicks = MinecraftServer.currentTick - this.lastTick;
+            if (elapsedTicks < 0) { // We should account for time drifting backwards
+                elapsedTicks = 1;
+            }
             if (d8 > 100.0D * (elapsedTicks == 0 ? 1 : elapsedTicks)) {
                 a.warning(this.player.name + " moved too quickly! Elapsed ticks: " +  (elapsedTicks == 0 ? 1 : elapsedTicks) + ", Distance change: " + d8);
                 this.disconnect("You moved too quickly :( (Hacking?)");