debian/changelog: Correct version number and formatting

The format of debian/changelog is very rigid as it is parsed by the
build tools to determine the version number and other information.
The version number must use ~ so that 1.3.0 is considered greater than
1.3.0~rc2 for upgrades, and there must be a Debian release number like
-1 because this is not a Debian-native package.  Finally, the default
urgency is medium these days.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>