Add travis build status image

Author: inexorabletash

README.md

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 Indexed DB
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+[![Build Status](https://travis-ci.com/w3c/IndexedDB.svg?branch=master)](https://travis-ci.com/w3c/IndexedDB)
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 This is a repository for the development of the Indexed Database (Third Edition) specification.
 * [Latest Editor's Draft](https://w3c.github.io/IndexedDB/)