[Bf-cycles] New CUDA Toolkit and support for Volta GPUs

[Thomas Dinges]

Hi everyone,

we are switching to the latest CUDA Toolkit 9.1. This enables support 
for nvidia Volta GPUs (sm_70).

Building with Toolkit 8.0 is still possible for the time being, but we 
recommend to update to 9.1.

The buildbot is already updated (Thanks to Sergey) and comes with 
support for Volta GPUs!

Best regards,

Thomas