Require explicit file format version in each manifest

The file format spec https://github.com/FamilySearch/gedcomx/blob/master/specifications/file-format-specification.md says:

* X-DC-conformsTo, used to identify the specification(s) this file conforms to. Conformance to http://gedcomx.org/file/v1 is assumed for all GEDCOM X files.

Rather than making that assumption, I'd think that the standard should require that the file format version be specified in every manifest.  Otherwise rolling out v2, etc, will be hard, since no one will be checking version numbers when they open files.

Side note: visiting the url http://gedcomx.org/file/v1 fails for me after redirecting to an https version, with "SSL connection error".  I'm not sure exactly what is expected to be there, and when.