Bugfix: path to the svg is absolute but doesn't include redmine's root

In redmine 2.3.1.stable, the path to the svg file was absolute but didn't include the path to redmine's root. This meant one would get "/graphs/target-version/1" instead of "/redmine/graphs/target-version/1". This commit fixes the bug by prepending Redmine::Utils.relative_url_root .