Back in Time release 1.1.16

Written for Back In Time by Germar on 2017-03-28

This is a bug-fix release for Back in Time. I pushed the last release a bit overhasty which broke automatic snapshots. Sorry

Changelog:
* backport bug fix: start a new ssh-agent instance only if necessary (https://github.com/bit-team/backintime/issues/722)
* Fix bug: OSError when running backup-job from systemd (https://github.com/bit-team/backintime/issues/720)