Add support for Mac in travis-tool.sh.
Add support for Mac in travis-tool.sh.

robzhu/r-travis,hadley/r-travis,qinwf/r-travis,eddelbuettel/r-travis,hadley/r-travis,eddelbuettel/r-travis,qinwf/r-travis,craigcitro/r-travis,craigcitro/r-travis,robzhu/r-travis