Fix assertion error in Ember for version downloads

When running locally, I get the following error on a crates page, and no
download graph gets rendered.

> Assertion Failed: You must include an 'id' for version-download in an
> object passed to 'push'

This has been happening since 3c92ca2
landed.

The fix involves telling Ember how to create a unique ID for a
version-download object, using the composite key.

Author: bryanburgers

app/serializers/version-download.js

@@ -0,0 +1,7 @@
+import DS from 'ember-data';
+
+export default DS.RESTSerializer.extend({
+    extractId(modelClass, resourceHash) {
+        return `${resourceHash.date}-${resourceHash.version}`;
+    },
+});