another attempt to make sure all packages are loadable
use the `separate` field to check some packages separately because they are time-consuming
for knitr, the two packages BTYD and R.rsp seem to be very time-consuming
update branch Thu Oct  2 02:29:02 CDT 2014
update branch knitr Thu Oct  2 13:02:26 CDT 2014
update branch knitr Thu Oct  2 14:49:45 CDT 2014
update branch knitr Thu Oct  2 19:41:50 CDT 2014