I don’t know if he answered you @aron, but I’m trying to use the same code and getting the same error. As a absolute newbie, I think I picked a project with a pretty steep learning curve and I don’t know which direction to look.
Same for me here 
Maybe the sparkfun tutorial will help?
https://learn.sparkfun.com/tutorials/wireless-arduino-programming-with-electric-imp
Hi @Johnygab / @Hugo - you could try my Imp based programmer that uses in-system programming (rather than requiring a bootloader) and will work with Arduino boards like the UNO, i.e. the ATmega328, or pretty much any other popular AVR MCU.
I posted about this a few days ago but doesn’t seem to have elicited too much notice 
Hope it’s of some use to you and others looking to program AVRs / Arduinos via the Imp.
http://forums.electricimp.com/discussion/2897/imp-powered-in-system-programmer-for-avr-mcus
I produced a huge writeup about it here:
Have fun and tell me how it goes 
/George
Hey guys, I just created a pull request on this amazing work that programs the Arduino. I have added Dropbox support using the dropins API so that you can utilize Dropbox for getting your Hex file. It is pretty slick as I added support to “remember” the last links using HTML5 storage and you can keep clicking “Upload” after the Dropbox sync has pulled your hex files over from your PC. Pretty slick!
If Sparkfun accepts the pull request, then you can have it too:
Here is my pull request:
Got it working!
this work very well : https://learn.sparkfun.com/tutorials/wireless-arduino-programming-with-electric-imp
BUT !!! your Atmega328p must have the Uno boot loader, otherwise its now waiting long enough in the Bl section and the imp is not able to get a reply.