R-related tests are failing

Disallowing sources: r-source
To add unlisted APT sources, follow instructions in https://docs.travis-ci.com/user/installing-dependencies#Installing-Packages-with-the-APT-Addon
0.42s$ curl -sSL "keyserver.ubuntu.com/pks/lookup?op=get&search=0x51716619E084DAB9" | sudo -E apt-key add -
OK
0.01s$ echo "deb http://cran.rstudio.com/bin/linux/ubuntu trusty/" | sudo tee -a ${TRAVIS_ROOT}/etc/apt/sources.list >/dev/null
0.40s$ curl -sSL "https://build.travis-ci.org/files/gpg/r-packages-trusty.asc" | sudo -E apt-key add -
OK
0.01s$ echo "deb http://cran.rstudio.com/bin/linux/ubuntu trusty/" | sudo tee -a ${TRAVIS_ROOT}/etc/apt/sources.list >/dev/null
0.51s