added build badge

Author: Alexandre Guidet

README.md

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 PhpDbMigration - full PHP database migration tool
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+[![Build Status](https://travis-ci.org/alwex/php-database-migration.svg?branch=master)](https://travis-ci.org/alwex/php-database-migration)
 [![Packagist](https://img.shields.io/packagist/dt/php-database-migration/php-database-migration.svg?maxAge=2592000)]()
 
 This is a full standalone PHP tool based on [Symfony Console](http://symfony.com/doc/current/components/console)