gnome15: switch dep from virtual/python-imaging to dev-python/pillow

[evadim]

during update, portage show warning:

The following mask changes are necessary to proceed: (see "package.unmask" in the portage(5) man page for more details) required by app-misc/gnome15-0.10.2::gnome15 required by @selected required by @world (argument) /usr/portage/profiles/package.mask: Justin Lecher (10 Nov 2015) Compatibility virtual for transition from dev-python/imaging to dev-python/pillow obsolete now #508266 =virtual/python-imaging-2

[CMoH]

Thanks - fixed in dd6628376d5d8af495f216ede5748c2b8c952316