Make "rel" required.

RFC 5988bis lists the link relation type as one of the three
required aspects of a link.

Since we do not need to support the deprecated "rev" mechanisms
for specifying relation types, we can make "rel" required.

This will encourage hyper-schema authors to think more about
choosing relation types.

Author: handrews

hyper-schema.json

@@ -14,7 +14,7 @@
         "linkDescription": {
             "title": "Link Description Object",
             "type": "object",
-            "required": [ "href" ],
+            "required": [ "href", "rel" ],
             "properties": {
                 "href": {
                     "description": "a URI template, as defined by RFC 6570",

jsonschema-hyperschema.xml

@@ -758,6 +758,7 @@
                     resource. The value MUST be a registered link relation from the
                     <xref target="RFC5988">IANA Link Relation Type Registry established in RFC 5988</xref>,
                     or a normalized URI following the <xref target="RFC3986">URI production of RFC 3986</xref>.
+                    This property is required.
                 </t>
 
                 <t>