Rename sourcesFile to sourcesJson

[cprussin]

See #159 (comment). I opted to rename the option rather than updating the docs because the cli option is --sources-json and so using an option sourcesJson is more obvious than keeping it as sourcesFile.

[nmattia]

Thanks! I pushed some CI fixes to your branch, the tests weren't being run on PRs of non-collaborators. As you can see the tests fail, the reason is that we version every change in sources.nix. I think the simplest solution is to keep the name sourcesFile and rename the CLI arg to --sources-file. That's also more in line with what we discussed in #159 since there'll soon be a single "sources" file (sources.nix will disappear).

[cprussin]

@nmattia done, let me know if this is what you had in mind

[nmattia]

@cprussin awesome, thanks!

[nmattia]

I've amended the CHANGELOG, hope you don't mind! I'll merge as soon as this is green.