Add travis shield to README.md.
Add travis shield to README.md.

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