Fixed resetting database in migrations tests

- due to schema changes between versions, we need to completely nuke the
  DB between these tests
- this is definitely not the best way to do it but I'll fix properly next week

ghost/core/test/integration/migrations/migration.test.js

@@ -31,6 +31,7 @@ describe('Migrations', function () {
 
     describe('Database initialization + rollback', function () {
         beforeEach(async function () {
+            await knexMigrator.reset({force: true});
             await knexMigrator.init();
         });