rubygem-goocanvas rubygem-gtk3 dependencies

[Tijl Coosemans]

Hi,

I think graphics/rubygem-goocanvas should depend on rubygem-gtk3 instead of
rubygem-gtk2.
And in x11-toolkits/rubygem-gtk3 it should say USE_GNOME=gtk30 instead of
USE_GNOME=gtk20.