ajaxorg/ace#1131 has been fixed, so revert commit 546712995c01dcab0754a7aef0e578065c76ad9a.
partially revert a73f7319e7a7066bb2909708bb089f6de2ccb28f if you call knit2html(), you must guarantee you have markdown installed

otherwise if R CMD build uses the knitr engine, it will fail to generate the HTML vignette but the user is unable to see what is wrong either (thanks, Dan Tenenbaum)
knitr imports markdown now because the markdown vignette engine really need it
use the svn revision number instead of R version number