Update index.md

Author: jpadilla

docs/index.md

@@ -1,6 +1,6 @@
 <div class="badges">
     <a href="https://travis-ci.org/GetBlimp/django-rest-framework-jwt">
-        <img src="https://travis-ci.org/GetBlimp/django-rest-framework-jwt.png?branch=master">
+        <img src="https://travis-ci.org/GetBlimp/django-rest-framework-jwt.svg?branch=master">
     </a>
     <a href="https://pypi.python.org/pypi/djangorestframework-jwt">
         <img src="https://img.shields.io/pypi/v/djangorestframework-jwt.svg">