docs: remove stale badge

Author: nodkz

README.md

@@ -6,7 +6,6 @@
 [![trends](https://img.shields.io/npm/dt/graphql-compose-mongoose.svg)](http://www.npmtrends.com/graphql-compose-mongoose)
 [![Join the chat at https://gitter.im/graphql-compose/Lobby](https://badges.gitter.im/graphql-compose/graphql-compose.svg)](https://gitter.im/graphql-compose/Lobby)
 [![Commitizen friendly](https://img.shields.io/badge/commitizen-friendly-brightgreen.svg)](http://commitizen.github.io/cz-cli/)
-[![Greenkeeper badge](https://badges.greenkeeper.io/graphql-compose/graphql-compose-mongoose.svg)](https://greenkeeper.io/)
 
 This is a plugin for [graphql-compose](https://github.com/graphql-compose/graphql-compose), which derives GraphQLType from your [mongoose model](https://github.com/Automattic/mongoose). Also derives bunch of internal GraphQL Types. Provide all CRUD resolvers, including `graphql connection`, also provided basic search via operators ($lt, $gt and so on).