Use GitHub API instead of Maven repo metadata and Aether to find new versions of Boot when upgrading with boot -u

[micha]

The current implementation depends on the behavior of Aether resolving the special RELEASE version, e.g.

[boot/core "RELEASE"]

This is problematic for a number of reasons:

• The RELEASE version doesn't allow restricting upgrades to a specific major version, which results in incompatible upgrades.
• The Aether machinery is complex and difficult to modify or extend.
• Aether caches metadata which prevents upgrading sometimes.

The solution: use the GitHub API to search for new versions of Boot, don't cache, and implement filtering of incompatible versions.

[micha]

superseded and fixed by #300