Restrict manifest v3 to 128.0b1 (Fixes #280)

[MelissaAutumn]

Fixes #280

This adds two additional checks to our manifest.json parser.

1. If a manifest.json has manifest_version: 3, require that strict_min_version exists and is greater than or equal to 128.0b1.
2. If a manifest.json has manifest_version: 3 don't allow the applications key.

cc @jobisoft