Update v2 context

This PR implements the extra changes as described in w3c/vc-bitstring-status-list#159 (comment), now that w3c/vc-bitstring-status-list#159 has been merged.

Author: iherman

contexts/credentials/v2

@@ -211,8 +211,14 @@
           "@type": "https://w3id.org/security#multibase"
         },
         "ttl": "https://www.w3.org/ns/credentials/status#ttl",
-        "statusReference": "https://www.w3.org/ns/credentials/status#statusReference",
-        "statusSize": "https://www.w3.org/ns/credentials/status#statusSize",
+        "statusReference": { 
+          "@id": "https://www.w3.org/ns/credentials/status#statusReference",
+          "@type": "@id"
+        },
+        "statusSize": {
+            "@id": "https://www.w3.org/ns/credentials/status#statusSize",
+            "@type": "https://www.w3.org/2001/XMLSchema#positiveInteger"
+        },
         "statusMessage": {
           "@id": "https://www.w3.org/ns/credentials/status#statusMessage",
           "@context": {