Treat version 0.x as unstable.

Per section 4 of semver version [2.0.0][1], "the public API should not
be considered stable".  Therefore, I argue that isStable should return
false for such a version.

This clause was also in semver version [1.0.0][2] in section 6.

[1]: http://semver.org/spec/v2.0.0.html "Semantic Versioning 2.0.0"
[2]: http://semver.org/spec/v1.0.0.html "Semantic Versioning 1.0.0"