set CRAN URL via environment variable RTRAVIS_CRAN_URL, default http://cran.rstudio.com

An "options" call is appended to `Rprofile.site` during bootstrapping
Merge pull request #32 from krlmlr/6-cran-url

set CRAN URL via environment variable RTRAVIS_CRAN_URL, default http://cran.rstudio.com