222.27.5.en.US OTA Update for MOTOX 1st Gen Least mFastboot needed.

I currently am on Android 5.1 system ver. 222.21.15.ghost_row.Retail.en.US

Currently I have TWRD fastboot installed but in order to perform a successful OTA update I must install mfastboot. Here is my question, where can I find the latest/compatible mFastboot img? The only copy I have is version 4.4.2 from way back when I was still running Kitkat.

Thanks.