libtorrent-rasterbar: updated to 1.2.15

1.2.15 released

cache DNS lookups for SOCKS5 proxy
fix stalled pieces on disk-full errors
fix build configuration issue on NetBSD, OpenBSD and DragonFly
make UTF-8 sanitization a bit stricter. This will re-write invalid UTF-8
code points encoding surrogate pairs
fix restoring last_seen_complete from resume data
fix issue on MacOS where the DHT was not restarted on a network-up notification
make remove_torrent flags be treated as flags (instead of an enum)

(adam)