Add Travis badge to README

Author: pglombardo

README.md

@@ -8,6 +8,8 @@ The instana package provides Python metrics and traces (request, queue & cross-h
 
 This package is currently in BETA (but maturing fast).
 
+[![Build Status](https://travis-ci.org/instana/python-sensor.svg?branch=master)](https://travis-ci.org/instana/python-sensor)
+
 ## Note
 
 This package supports Python 2.7 or greater.