bump roxygen2 and formatR versions
roxygen2 4.0 changes
remove the old LaTeX vignette
Revert "do not check vignettes"

This reverts commit 4ac275c59aee48f31d7d3a3169e871b0961138d4.

We use HTML vignettes now, so no dependency on LaTeX packages
add the new R Markdown vignette
require knitr > 1.5 to build the Rmd vignette, which needs to be installed from RForge.net at the moment
bump version