Mark 2.4 as "UNRELEASED" in debian/changelog.

At this stage, there's no tagged snapshot for 2.4 so building the
'testing' branch will fail with:

    E: Failed getting release file http://tagged.snapshots.deb.tails.boum.org/2.4/debian/dists/jessie/Release

In order to build the 'testing' branch to record the .build-manifest
and SquashFS sort file, I'll have to make it UNRELEASED, temporarily.