Update build status icon on github pages page.

docs/index.md

@@ -4,7 +4,7 @@
 <a href="https://twitter.com/share" class="twitter-share-button" data-url="django-rest-framework.org" data-text="Checking out the totally awesome Django REST framework! http://www.django-rest-framework.org" data-count="none"></a>
 <script>!function(d,s,id){var js,fjs=d.getElementsByTagName(s)[0];if(!d.getElementById(id)){js=d.createElement(s);js.id=id;js.src="http://platform.twitter.com/widgets.js";fjs.parentNode.insertBefore(js,fjs);}}(document,"script","twitter-wjs");</script>
 
-<img src="https://secure.travis-ci.org/tomchristie/django-rest-framework.png?branch=master" class="travis-build-image">
+<img src="https://secure.travis-ci.org/tomchristie/django-rest-framework.svg?branch=master" class="travis-build-image">
 </p>
 
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