let's do something crazy: see if travis can afford 30 parallel builds
unique package names
install deb dependencies of rmarkdown if available
when apt-get install fails, re-run it with stdout to see what is going on
see why rJava fails to install
add more broken packages in PPA
add some packages that fail to install, and output to stdout for them
some packages are time-consuming, so enable stdout to avoid travis timeouts
tweak error message
arrange packages in the build matrix alphabetically

at some point, we need to figure out a better way to arrange them, e.g. they should be balanced according to the CRAN check time, so that each item in the matrix uses more or less the same time, otherwise time-consuming packages might be arranged in the same item
add more packages that failed to install for unknown reasons