install rmarkdown when checking knitr's reverse dependencies
recursively figure out the _required_ dependencies of _all_ dependencies of p
filter the check logs, and remove the OK lines
apt-get build-dep the broken packages if possible
factor out the procedure to install R packages into apt_get() so I do not have to remember intersect(tolower()) every time
print out the package names below maintainer
let's do something crazy: see if travis can afford 30 parallel builds
update branch Thu Oct  2 02:29:02 CDT 2014
update branch knitr Thu Oct  2 13:02:26 CDT 2014