Enable subscriptions in GraphQLModule

Author: cschroeter

sample/12-graphql-apollo/src/app.module.ts

@@ -7,6 +7,7 @@ import { CatsModule } from './cats/cats.module';
     CatsModule,
     GraphQLModule.forRoot({
       typePaths: ['./**/*.graphql'],
+      installSubscriptionHandlers: true
     }),
   ],
 })