Update to latest ggplot2 release once it contains our patches

The two patches are (1) drawing a ribbon only for intervals with non-NA values (https://stat.ethz.ch/pipermail/r-help/2010-June/241559.html) and (2) positioning a legend at the top of a graph (https://stat.ethz.ch/pipermail/r-help/2010-June/241618.html).