Fix update logic

[Fábio Rosado]

When moving opsdroid-web to v2 I implemented the same logic as we used to have in v1. Although, the logic only works for releases that we do with github. This is because we are hitting a github endpoint to check the latest release.

With opsdroid-desktop, the release was done with GitHub since it is an electron app, but opsdroid-web is released to npm so the endpoint always hits a 404 (because we don't have any release in github).

We should update the logic to check npm instead of Github so the update message works