Increase the resistance of a photoresistor with increase in light intensity
Arduino can not measure resistance. It can measure voltage.
To measure resistance you can convert resistance to voltage using a voltage divider.  

In Arduino world a voltage divider consists out of 2 resistors in series between 5V and gnd. The connection to the 2 resistors is connected to a analogue pin. 
Depending whether your LDR is connected to gnd or 5V the measured value on the analogue pin will go up or down with the resistance of the LDR.  

In other words swap the resistor and your LDR from position.