Pro-Tip: Raspbian comes with loads of bloatware. If you really need a headless server go ahead and install Ubuntu Server 14.04... IMO much better than Snappy Ubuntu Core
UbuntuSC requires ARMv7 ISA CPU, which among Raspberry Pi models is only featured on the BCM2836/BCM2709 SoC in "Rasbperry Pi 2 Model B" (note the "2").
DietPi http://dietpi.net/
Minibian https://minibianpi.wordpress.com/
Both Raspbian-based
And, in just 80MB, Slitaz-ARM http://arm.slitaz.org/
Though I don't know if the latter has SSH enabled by default.
I used to use 'Diet Raspbian,' which took plain Raspbian and stripped it down, but this is a much nicer alternative, since it's an official build!