Tell Travis to build vignettes

Currently, the Travis build will fail with ``WARNINGS_ARE_ERRORS`` set to ``1`` as there is no PDF version of the vignette in the package and one will not be created with the default build arguments supplied by [r-travis](https://github.com/craigcitro/r-travis). This changes the ``R CMD build`` arguments to ensure the vignette is built, and so the Travis build passes.