Fix redis PPA for `language: csharp` (#960)
Fix redis PPA for `language: csharp` (#960)

Fix redis PPA. See travis-ci/travis-ci#7332.
JuliaCI/travis-build,kevmoo/travis-build,craigcitro/travis-build,andyli/travis-build,andyli/travis-build,craigcitro/travis-build,JuliaCI/travis-build,kevmoo/travis-build