we sadly have a serious issue with the stable pi images, the broadcom bootloader will refuse to boot with a newer kernel and there is no mechanism yet to update the gadget snap. if you installed from a stable image you will be stuck until gadget updates are implemented (there is a snappy sprint this week where exactly this will be worked on but i assume this will still take a bit to actually land).
one way to get a new kernel (and gadget) as a workaround is to use a daily built image from http://people.canonical.com/~ogra/snappy/all-snaps/daily/
which builds from the edge channel, will get daily updates and comes with a newer gadget snap.
to keep the rootfs on stable you can “snap refresh core --stable” after installation, that way you get the new gadget, regular kernel updates and a stable rootfs.
sorry for the inconvenience …