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.

Powered by [r-travis](https://github.com/craigcitro/r-travis).

Note: really have to install LaTeX for testing the package.