Change coverall link to point to master branch

README.md

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 # Firebase Authentication JWT Verifier
 
 [![Build Status](https://travis-ci.org/LewisWatson/firebase-jwt-auth.svg?branch=master)](https://travis-ci.org/LewisWatson/firebase-jwt-auth)
+[![Coverage Status](https://coveralls.io/repos/github/LewisWatson/firebase-jwt-auth/badge.svg)](https://coveralls.io/github/LewisWatson/firebase-jwt-auth)
 [![GoDoc](https://godoc.org/github.com/SermoDigital/jose?status.svg)](https://godoc.org/github.com/LewisWatson/firebase-jwt-auth)
-[![Coverage Status](https://coveralls.io/repos/github/LewisWatson/firebase-jwt-auth/badge.svg?branch=feature%2Fexpand-tests)](https://coveralls.io/github/LewisWatson/firebase-jwt-auth?branch=feature%2Fexpand-tests)
 [![stability-unstable](https://img.shields.io/badge/stability-unstable-yellow.svg)](https://github.com/emersion/stability-badges#unstable)
 
 This library follows the instructions described in [verify id tokens using third-party JWT library](https://firebase.google.com/docs/auth/admin/verify-id-tokens#verify_id_tokens_using_a_third-party_jwt_library) section of the firebase documentation.