add travis-tool.sh to package's .Rbuildignore during bootstrap

The name of the travis-tool.sh script should be opaque to the user,
and the script is not even part of the package repository.
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 #33 from krlmlr/rbuildignore

add travis-tool.sh to package's .Rbuildignore during bootstrap
Merge pull request #32 from krlmlr/6-cran-url

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