enable continuous integration via craigcitro/r-travis

This enables support for continuous integration via Travis CI.
By [flipping a switch](https://travis-ci.org/profile),
each pushed revision will be checked with R CMD build and R CMD install --as-cran.
Currently, the checks [fail](https://travis-ci.org/krlmlr/RTikZDevice/builds/13174371),
but this will hopefully change soon.

Note: really have to install LaTeX for testing the package.