Update to sagemath 6.8 in rawhide

[Paulo César Pereira de Andrade]

Hi,

I will update sagemath to version 6.8 today.
  I should update only pynac as dependency.

The update took more time than I expected, due
to debugging build failures, some that took really
quite a lot of time, to come with a 1 or 2 lines patch :(

The package should be at least as functional as the
6.5 version.

After updating to 6.8, that requires a bundled pari 2.8,
I will work on updating to 6.9.

Thanks,
Paulo