Fixing the travis build svg to amster

Author: joaomdmoura

README.md

@@ -1,6 +1,6 @@
 # ActiveModel::Serializer
 
-[![Build Status](https://travis-ci.org/rails-api/active_model_serializers.svg)](https://travis-ci.org/rails-api/active_model_serializers)
+[![Build Status](https://travis-ci.org/rails-api/active_model_serializers.svg?branch=master)](https://travis-ci.org/rails-api/active_model_serializers)
 <a href="https://codeclimate.com/github/rails-api/active_model_serializers"><img src="https://codeclimate.com/github/rails-api/active_model_serializers/badges/gpa.svg" /></a>
 <a href="https://codeclimate.com/github/rails-api/active_model_serializers/coverage"><img src="https://codeclimate.com/github/rails-api/active_model_serializers/badges/coverage.svg" /></a>