Removed bookshelf-relations config from PostRevision model

This was causing issues with saving posts when revisions did not have authors
and is not necessary for the functionality we desire.

ghost/core/core/server/models/post-revision.js

@@ -4,8 +4,6 @@ const _ = require('lodash');
 const PostRevision = ghostBookshelf.Model.extend({
     tableName: 'post_revisions',
 
-    relationships: ['author'],
-
     author() {
         return this.belongsTo('User', 'author_id');
     },