bumping the pulseaudio requirement

[Matthias Clasen <matthias clasen gmail com>]

We have a nice sound panel simplification incoming in
https://bugzilla.gnome.org/show_bug.cgi?id=674831 which relies on pa
2.0 api to get rid of the 'Hardware' tab.

I propose that we bump the pulseaudio requirement to 2.0 so we can
land this nice improvement. Pulseaudio 2.0 was released 2 month ago,
which is admittedly fairly recent, but I suspect that 1.99.1 from
March has all the required API already.

Matthias