Nightly builds #1433

It would be great if Travis had another step that would build VersionPress and publish it somewhere so that:

• Each build is uniquely identified, which our npm run build script already does. It creates something like versionpress-4.0-beta2-9-gd9a03c79.zip, which means "the closest tag is 4.0-beta2, there have been 9 commits since then and the build was from commit d9a03c79".
• When in a branch / PR, the most recent build would be aliased as https://some-service.com/versionpress-branch-name.zip
• Latest builds from master would be aliased as https://some-service.com/versionpress-master.zip or .../versionpress-nightly.zip.

That's the best setup but whatever gives us versionpress-nightly.zip on some known URL would already be quite useful.