Update value.py

In def decrease() function the following lines was copied from increase function
if valuei.currentValue >= valuei.minimumValue:
                raise LdtpServerException('Maximum limit reached')

hence the function was always throwing exception when you try to decrease the value of slider/spin control so changed to 
if valuei.currentValue <= valuei.minimumValue:
                raise LdtpServerException('Minimum limit reached')