fix commented documentation of env in sample file; add example for build matrix
fix commented documentation of env in sample file; add example for build matrix

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