Bug 15303 Followup: If install LE dependencies from backports on Jessie

Bug 15303 Followup: If install LE dependencies from backports on Jessie

On Debian Jessie, apt needs to be told it should get the dependencies
for the LE package from backports, or it will error. We check if we are
on Jessie and do that automatically so users don't need to set priorities
manually in apt.

Changed to lsb_release -c -s.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>