generalizing travis.sh for linux/macos-based ci hosts

[guillermooo]

There are more SaaS CI servers today than a few years ago. While so far I've seen each CI environment can be adapted so that the travis.sh script (and also appveyor.ps1) can be used with them, it should be possible to further decouple the travis.sh script from the Travis-CI specifics.

[randy3k]

Agree. That’s the reason I have separated the installing sublime text script. And we need to also make sure the refactoring won’t break the usages of existing packages.