ApproximateQuantilesCombineFn uses deprecated '-' operator

This causes a TypeError with numpy 1.16.4

apache_beam/transforms/stats.py", line 437, in <lambda>
{{ else (a > b) - (a < b)}}
TypeError: numpy boolean subtract, the `-` operator, is deprecated, use the bitwise_xor, the `^` operator, or the logical_xor function instead.