Add Travis badge

Signed-off-by: Alex Ellis <[email protected]>

README.md

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 vcenter-connector is an OpenFaaS event connector built to consume events from vCenter.
 
+[![Build Status](https://travis-ci.org/openfaas-incubator/vcenter-connector.svg?branch=master)](https://travis-ci.org/openfaas-incubator/vcenter-connector)
+
+## Overview
+
 With this project your functions can subscribe to events generated by the changes (i.e. events) in your vCenter installation - for instance a VM being created, turned on or deleted. This allows you to extend vCenter's functionality by writing functions to execute each time an event is fired. An example may be tagging a VM with the date it was last turned on or applying a tag showing which user made a change to an object. 
 
 ## Status