Hello Guix!

This is a series of two patches that will update OpenColorIO to v2.0.0,
which was released 3 days ago. The first, in this message, will add a
dependency for OCIO 2.0.0 and the second, which will be sent soon, will
actually update the package.

Please let me know if I should make any changes.

Thanks,
-- Evan Straw <evan.straw99@gmail.com>