build: require NUnit 2.5, for real
build: require NUnit 2.5, for real

Oops, the fundamental version change was missed in previous
big commit, which explains the rationale:

https://git.gnome.org/browse/banshee/commit/?id=52166da19192db39f8846c7398b512347173e37e

GNOME/banshee,stsundermann/banshee,babycaseny/banshee,Carbenium/banshee,stsundermann/banshee,GNOME/banshee,Carbenium/banshee,ixfalia/banshee,GNOME/banshee,arfbtwn/banshee,stsundermann/banshee,ixfalia/banshee,ixfalia/banshee,arfbtwn/banshee,arfbtwn/banshee,babycaseny/banshee,Carbenium/banshee,arfbtwn/banshee,GNOME/banshee,babycaseny/banshee,ixfalia/banshee,arfbtwn/banshee,stsundermann/banshee,stsundermann/banshee,ixfalia/banshee,arfbtwn/banshee,babycaseny/banshee,GNOME/banshee,babycaseny/banshee,GNOME/banshee,Carbenium/banshee,babycaseny/banshee,stsundermann/banshee,arfbtwn/banshee,GNOME/banshee,babycaseny/banshee,stsundermann/banshee,ixfalia/banshee,ixfalia/banshee,Carbenium/banshee,Carbenium/banshee,arfbtwn/banshee,ixfalia/banshee,GNOME/banshee,stsundermann/banshee,babycaseny/banshee