skype: add workaround for pulseaudio glitches

See http://arunraghavan.net/2013/08/pulseaudio-4-0-and-skype/

Relevant bug reports:
  https://github.com/NixOS/nixpkgs/issues/788
  https://bugs.archlinux.org/task/35690
  https://bugs.freedesktop.org/show_bug.cgi?id=50510

We don't want to disable pulseaudio timer-based scheduling (tsched=0)
because that affects everyone. It seems this is the proper distro fix
for skype + pulseaudio at the moment.