fix: ensure a trailing slash in log dir name

LOGDIR="/foo/bar" (without  a trailing slash) produces /foo/barbackup.lock and/foo/barduplicity-* files, not /foo/bar/backup.lock and /foo/bar/duplicity-* files as expected. This fix ensures a trailing slash in the log dir name.
Merge pull request #71 from martinec/master

fix: ensure a trailing slash in log dir name