[chore, CI] .travis.yml: update Docker images (#642)
[chore, CI] .travis.yml: update Docker images (#642)

See https://github.com/koreader/koreader-base/pull/640#issuecomment-379771447
Frenzie/koreader-base,houqp/koreader-base,koreader/koreader-base,Frenzie/koreader-base,Frenzie/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,houqp/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,NiLuJe/koreader-base,NiLuJe/koreader-base,houqp/koreader-base,houqp/koreader-base,koreader/koreader-base