Add Travis badge

Author: Caitlin Bales (MSFT)

README.md

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 *This SDK is currently in preview, please continue to provide [feedback](https://github.com/microsoftgraph/msgraph-sdk-php/issues/new) as we iterate towards a production supported library.*
 
+[![Build Status](https://travis-ci.org/microsoftgraph/msgraph-sdk-php.svg?branch=master)](https://travis-ci.org/microsoftgraph/msgraph-sdk-php)
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 ## Installation
 The PHP SDK can be installed with Composer:
 ```