install binary R packages with just one apt-get install call -- again

- otherwise the logs just look horrible: e.g.,
https://travis-ci.org/yihui/tikzDevice/builds/13447667, lines 365-700
- runtime penalty is substantial, too

Compare with run using this branch: https://travis-ci.org/yihui/tikzDevice/builds/13448326. It's about 30-60 secs faster, and the logs are much nicer.