gnulib-tool: adjust translation wget to avoid a https redirection
Context: http://translationproject.org/latest/gnulib redirects to
https://translationproject.org/latest/gnulib/
Rationale: if the user falls back to wget, she doesn't have rsync and
is probably in a minimal build environment, where packages such as
'ca-certificates' are missing as well, resulting in a failed (and
difficult to detect since ignored) translation initial fetch.
Consequently let's avoid https if possible, and add the missing
trailing slash. This also avoids an unnecessary 302 redirection.
* gnulib-tool: Add trailing slash to gnulib URL.