test against multiple versions of R

[chriddyp]

#254 was discovered today against R 3.1.3. We should test against multiple versions of R releases to avoid this in the future. It looks like this repo is a good start at this: https://github.com/metacran/r-builder

[cpsievert]

CRAN provides checks for multiple versions of R and different OSes -- https://cran.r-project.org/web/checks/check_results_plotly.html

IMO, now that we're on CRAN, setting this up on Travis would be more trouble than its worth