Add some tests on Travis CI
Let's add some tests on Travis CI, e.g.,
- AppImageUpdate tries to update itself (using an outdated version of itself)
- AppImageUpdate tries to update itself (using the recent version of itself)
- AppImageUpdate tries to update another application that was only half-downloaded
Hm, I'd rather implement something standalone, e.g., setting up a local HTTP server using python3 -m http.server, and testing against that. I don't really want to introduce a dependency on GitHub or Bintray or whatever service an update information parser is implemented for. Maybe we can even simulate conditions like they're required e.g. for gh-releases-zsync by hacking /etc/hosts.
Please add the rewrite milestone rather than tagging the title with [rewrite], thanks.
I'd be fine with being pragmatic here and testing against real-world services. For me it would be sufficient if the tests can be executed on Travis CI against real-world GitHub Releases, Bintray, OBS etc. hosts.