Warn about disallowed APT sources (#531)
Warn about disallowed APT sources (#531)

* [skip ci] Warn about disallowed APT sources

* update APT sources note in travis-build to point to `sudo: required`

* Fix apt addon message

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