What is the relation of stall speed to the position of the center of pressure with respect to the centre of gravity?
The comment written by wbeard52 is correct however this does still not explain exactly why the stall speed is increased. As an airframe stalls at an angle of attack, which will result in CLmax. 



If the CG is moved to a more forward position, more downward force will be required on the horizontal stabilator to counteract the higher moment of the lift generated by the wings. This will increase the apparent weight. As (apparent) weight is increased stall speed will increase. All other parameters are fixed, in this situation. 

Stall speed formula : V = √( 2 W g / ρ S CLmax )