Remove IO::Async from t_hangs.yml

Newer versions have a 10-second alarm() which should guard against hanging, tested 0.59 on perl-5.16.3 on Linux/x64 and perl-5.18 on FreeBSD 9.0 without problems. Suspect an older version of this distribution was hanging on some systems but it should be more stable these days.
Merge pull request #1 from tm604/patch-1

Remove IO::Async from t_hangs.yml