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I did some code investigation and found the following explanation. The changes were introduced in trac ticket 23338, and you can see in the changes made on the polynomial_ring_constructor.py function, that before, your k was tested to be an integer (see the isinstance(arg1, integer_types + (Integer,)) tests), this is why you get an error since your k is a rational, not an integer:

sage: k
6
sage: k.parent()
Rational Field
sage: from six import integer_types
sage: isinstance(k, integer_types + (Integer,))
False

In the recent implementation, your k is transformed into an integer, see k = Integer(arg), this is why your code works on recent versions of Sage.

So, though i encourage you to upgrade your Sage installation, you can also transform your k into an integer:

sage: k = ZZ(n*(n-1)/2)

or

sage: k = Integer(n*(n-1)/2)