Confusion regarding step responses of integrator and differentiator?
Unless your step is negative, I'm going to surmise that your integrator model is an inverting configuration, which is why your ramp goes negative.  Maybe resistor to negative op amp input, capacitor from same input to output?  In your code, it's expressed by the negative sign on line 7.

For the differentiator, if you're using ideal models, the response to a step function is a Dirac delta function, which is infinite at t=0 and zero otherwise.  An ideal differentiator has a single zero at 0,0 (or 1,0 in Z space). 
 It appears that the package you're using is not designed for models with more zeros that poles.  In any case, the point at (0, infinity) will probably be an issue.