python3-systemd dependency

[J-F Mammet]

Freshly install fail2ban wouldn't work with my just upgraded to 15.04 system, as it requires the python3-systemd package to be able to use the systemd backend. I figure having this package as a dependency of fail2ban would help a lot of people who might wonder why they get some rather cryptic errors in the logs.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: fail2ban 0.9.1-1
ProcVersionSignature: Ubuntu 3.19.0-15.15-generic 3.19.3
Uname: Linux 3.19.0-15-generic x86_64
ApportVersion: 2.17.2-0ubuntu1
Architecture: amd64
CurrentDesktop: XFCE
Date: Sun Apr 26 18:06:29 2015
InstallationDate: Installed on 2012-11-10 (897 days ago)
InstallationMedia: Cubuntu 12.10 64bits v9 - Release amd64
PackageArchitecture: all
SourcePackage: fail2ban
UpgradeStatus: Upgraded to vivid on 2015-04-26 (0 days ago)
modified.conffile..etc.fail2ban.jail.conf: [modified]
mtime.conffile..etc.fail2ban.jail.conf: 2015-04-26T17:56:04.377099