#39 Fix for get_inter_arrival_time

This is the small fix for the pkt-index issue we briefly talked about in our last meeting. The commit in scapy which broke the code can be found [here](https://github.com/phaethon/scapy/commit/2a5b59e7ace8f342a4c46246d4b39efe308b57cc#diff-fb7023a8077b5e1da2c3a462e59e0a89). While the change wasn't (as far as I could see) properly documented anywhere, I think it's safe to adapt our code to the new version, since it has shipped this way since v0.22 back in November.