fix using big rational #**with unsigned ints

[stakach]

the compiler complains that -other is not possible when passing an unsigned value to the function

    if other < 0
      return (self ** -other).inv
    end

[straight-shoota]

I think this could be fixed simply by replacing -other with other.abs. That's done in other places as well. No need for an extra overload.

[straight-shoota]

Thank you @stakach