Readme: Use an SVG badge.

They look better on retina screens and when zoomed in.

README.md

@@ -5,7 +5,7 @@ Composer helps you declare, manage and install dependencies of PHP projects, ens
 
 See [https://getcomposer.org/](https://getcomposer.org/) for more information and documentation.
 
-[![Build Status](https://secure.travis-ci.org/composer/composer.png?branch=master)](http://travis-ci.org/composer/composer)
+[![Build Status](https://travis-ci.org/composer/composer.svg?branch=master)](https://travis-ci.org/composer/composer)
 
 Installation / Usage
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