Problem: data.json is a generated file but included in source control

Source control is for source files. `data.json` is not a source file so it should not be under source control, it should not be commited.

To make `data.json` available despite not being present in the repository, it should be handled as a build artifact. Build artifacts can be downloaded from github releases. This move needs a working travis CI configuration. I plan to create a PR for that.