Google publishes prebuilt tensorflow whl for python 3.4, 3.5, 3.6,
but nix expression for tensorflow only supported 3.5.
This change adds support for python-3.6.
When using cuda, the rpath was set to include GCC lib version 4.9.
I am not sure what this was attempting to do, but an effect was to
prevent certain python libraries to find the correct (newer) version
of the std lib.
Also avoid mentions of any specifc version in the
propagatedBuildInputs
