Have the same issue as you:
SPIFFSImpl: allocating 512+180+1400=2092 bytes
SPIFFSImpl: mounting fs @6b000, size=4000, block=1000, page=100
SPIFFSImpl: mount rc=-1
SPIFFSImpl: mounting fs @6b000, size=4000, block=1000, page=100
SPIFFSImpl: mount rc=0
As I am too stupid, to compile my own mkspiffs, I will have to wait until igrr will build a new exe.
In the Arduino/ESP8266 package there is the same file. Has this to be changed too?
ESP8266 Arduino add-on
Arduino IDE 1.6.5