Update to work with bittorrent-protocol@1.5.0

We can’t rely on Function.name to determine the name of the extension,
so we need to set it on the prototype.

See:

https://github.com/feross/bittorrent-protocol/commit/0b0942492adcd0ec22b
f4fa05259faf4d0191a28
Merge pull request #2 from feross/master

Update to work with bittorrent-protocol@1.5.0