riscof_model.py template: Use "-k" (--keep-going) when invoking make.

Make is primarily used as a parallel job dispatcher here. Without the -k
flag, a failing test case early in the run prevents others from being
scheduled and dispatched at all. We then get the following:

	INFO | Running Tests on Reference Model.
	INFO | Initiating signature checking.
	ERROR | Signature file : /path/to/DUT-thing.signature does not exist

...and no report is generated.

With "-k", make continues to schedule and dispatch test cases even after
one fails. Fails are determined by comparing signatures against
references anyways.

Fixes #73.