Add Travis badge to README

Author: epels

README.md

@@ -2,6 +2,8 @@ MessageBird's REST API for Python
 =================================
 This repository contains the open source Python client for MessageBird's REST API. Documentation can be found at: https://developers.messagebird.com/.
 
+[![Build Status](https://travis-ci.org/messagebird/python-rest-api.svg?branch=master)](https://travis-ci.org/messagebird/python-rest-api)
+
 Requirements
 ------------
 - [Sign up](https://www.messagebird.com/en/signup) for a free MessageBird account