Some modifications were in order so that gitlab-mirrors
works with the latest pyapi-gitlab.

This fixes issue #2 reported by @lmakarov.

Fixes issue #2

Update installation.md to include adding group to gitlab

Note about "Mirrors" group creation step in gitlab

If run in a cron job and gitlab is off-line update_mirror.sh
would turn the SVN repository into a bare repository.  This
then breaks the SVN mirror until bare is set back to false.

Major bugfix a working directory bug where
crontab execution fails.

This way users can utilize gitlab-mirrors as well as Administrators.

For installing prerequisites the python-gitlab doc was missing
the install command.

There's no need to move the project to a group if the user is
creating projects in their own namespace.