README: use circleci build badge

README.md

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-# graphql [![Build Status](https://travis-ci.org/graphql-go/graphql.svg)](https://travis-ci.org/graphql-go/graphql) [![GoDoc](https://godoc.org/graphql.co/graphql?status.svg)](https://godoc.org/github.com/graphql-go/graphql) [![Coverage Status](https://coveralls.io/repos/graphql-go/graphql/badge.svg?branch=master&service=github)](https://coveralls.io/github/graphql-go/graphql?branch=master) [![Join the chat at https://gitter.im/graphql-go/graphql](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/graphql-go/graphql?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+# graphql [![CircleCI](https://circleci.com/gh/graphql-go/graphql/tree/master.svg?style=svg)](https://circleci.com/gh/graphql-go/graphql/tree/master)[![GoDoc](https://godoc.org/graphql.co/graphql?status.svg)](https://godoc.org/github.com/graphql-go/graphql) [![Coverage Status](https://coveralls.io/repos/graphql-go/graphql/badge.svg?branch=master&service=github)](https://coveralls.io/github/graphql-go/graphql?branch=master) [![Join the chat at https://gitter.im/graphql-go/graphql](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/graphql-go/graphql?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 An implementation of GraphQL in Go. Follows the official reference implementation [`graphql-js`](https://github.com/graphql/graphql-js).