Fix libtorrent compilation by:
* Disable ipv6
* Remove the problematic patches/120-... ipv6 hack at the same time,
  as it is a 5-year old hack to fix one ipv6 problem.
* Update libtorrent to git master HEAD (0.13.6).

Libtorrent compilation has been broken due to patches/120-...
since #1181 got merged and ipv6 got enabled.
Users have seen issues like #1316 and #1804

IPv6 support in libtorrent master is not compelete.
Instead there is a separate ipv6 branch, which still needs some
cleanup before mainstream use. See discussion at
https://github.com/rakshasa/rtorrent/issues/59#issuecomment-56651538
So, it makes no sense to use ipv6 with the master branch.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>