Allow base context alone

The context cardinality test specifies 'one or more URIs' but rejects
the case of one URI. Change it to allow the one-URI case.

Fixes w3c#96

Author: clehner

test/vc-data-model-1.0/10-basic.js

@@ -33,7 +33,7 @@ describe('Basic Documents', function() {
       const doc = await util.generate('example-1.jsonld', generatorOptions);
       doc.should.have.property('@context');
       doc['@context'].should.be.a('Array');
-      doc['@context'].should.have.length.greaterThan(1);
+      doc['@context'].should.have.length.greaterThan(0);
     });
 
     it('MUST be one or more URIs (negative)', async function() {

test/vc-data-model-1.0/input/example-1-bad-cardinality.jsonld

@@ -1,6 +1,5 @@
 {
   "@context": [
-    "https://www.w3.org/2018/credentials/v1"
   ],
   "id": "http://example.edu/credentials/58473",
   "type": ["VerifiableCredential", "AlumniCredential"],