require(lattice) in vignette instead of depends

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/lme4@1882 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
bump version

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/lme4@1883 edb9625f-4e0d-4859-8d74-9fd3b1da38cb