Add trailing single quote to R command
Add trailing single quote to R command

See https://github.com/travis-ci/travis-build/commit/fb0a1e8d7b8065a7878ce97323a34f19a8a5b3f0

craigcitro/travis-build,JuliaCI/travis-build,craigcitro/travis-build,JuliaCI/travis-build,andyli/travis-build,andyli/travis-build,kevmoo/travis-build,kevmoo/travis-build