Set repo as default serializer only on repo serializer, not v3

app/adapters/repo.js

@@ -1,6 +1,8 @@
 import V3Adapter from 'travis/adapters/v3';
 
 export default V3Adapter.extend({
+  defaultSerializer: '-repo',
+
   buildUrl(modelName, id, snapshot, requestType, query) {
     var url = this._super(...arguments);
 

app/adapters/v3.js

@@ -6,8 +6,6 @@ export default DS.RESTAdapter.extend({
   auth: Ember.inject.service(),
   host: config.apiEndpoint,
 
-  defaultSerializer: '-repo',
-
   sortQueryParams: false,
   coalesceFindRequests: false,
   headers: {