Update .travis.yml
Update .travis.yml

r_github_packages -> r_packages
CRAN mirror cloud.r-project.org (more stable)
jangorecki/data.table,Rdatatable/data.table,Rdatatable/data.table,vlulla/data.table,vlulla/data.table,Rdatatable/data.table,vlulla/data.table,jangorecki/data.table,vlulla/data.table,jangorecki/data.table,Rdatatable/data.table,jangorecki/data.table