$ sudo ./ubuntu-device-flash core rolling --channel edge --kernel canonical-pi2-linux.canonical --gadget snap-src/*.snap --enable-ssh  --developer-mode --os ubuntu-core.canonical -o screenly.img

Determining gadget configuration
57.80 MB / 57.80 MB [=================================================================================================================================================================================================] 100.00 % 2.96 MB/s 19s
Installing snap-src/screenly-oem-pi2_2.4.3_all.snap
snap-src/screenly-oem-pi2_2.4.3_all.snap failed to install: open /tmp/diskimage228832684/system/etc/udev/rules.d/80-snappy_screenly-oem-pi2_screenly-client.sideload.rules.icXyYtbFjGiD: read-only file system