Validate version in the XSD

[tomwetjens]

Related to #187

The XSD has a type specified for the Version attribute:
{code}
<xs:simpleType name="SpecVersion">
<xs:restriction base="xs:string">

<xs:pattern value="(\d+.\d+.\d+)"/>
</xs:restriction>
</xs:simpleType>
{code}

We could actually let the XSD enforce that a message with a compatible version is being validated.
E.g. at least a major version, maybe even minor or patch versions. This would also 'document' which versions are compatible with the XSD.

[Albertwaninge-2]

Sounds good to validate on major version (semver rules)