at most 7 packages per job by default
sometimes `travis logs` does not work, so add an optional build number to use wget to download logs
re-check these package for vignette problems
yihui updated branch pkg/formatR Fri Oct 10 13:04:31 CDT 2014
yihui updated branch pkg/knitr Fri Oct 10 03:00:08 CDT 2014
yihui updated branch pkg/knitr Fri Oct 10 11:41:53 CDT 2014
yihui updated branch pkg/knitr Fri Oct 10 13:04:32 CDT 2014