Gemfile: update rbpdf version to 1.18.3 (#16040, #18586, #10681)

git-svn-id: http://svn.redmine.org/redmine/trunk@13726 e93f8b46-1217-0410-a6f0-8f06a7374b81
German translation updated by Daniel Felix (#18562)

git-svn-id: http://svn.redmine.org/redmine/trunk@13727 e93f8b46-1217-0410-a6f0-8f06a7374b81
German translation updated by Daniel Felix (#18563)

git-svn-id: http://svn.redmine.org/redmine/trunk@13728 e93f8b46-1217-0410-a6f0-8f06a7374b81