git-flow-log: Remove bashisms

Do not use [[, because that is a bashism, and breaks `git flow log` when
running under a /bin/sh that is not bash (for example, on Debian). Fixes
Debian #765520.

Reported-by: Paul van Tilburg <paulvt@debian.org>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Update changelog
Update of the contributers.
Version bump 1.9.0-dev.5
Merge branch 'feature/remove-bash-code' into develop