up the xz requirement to 5.0.3, as there are known issues with 5.0.2.
Simon supplies 5.0.3 for OS X, and most Linux distros are on 5.1.?alpha


git-svn-id: https://svn.r-project.org/R/trunk@63234 00db46b3-68df-0310-9c12-caf00c1e9a41