Update badges (#549)

* Use SVG build badge
* Add supported Python versions badge

[ci skip]

Author: hugovk

README.rst

@@ -1,12 +1,18 @@
-.. image:: https://secure.travis-ci.org/pytest-dev/pytest-django.png?branch=master
-   :alt: Build Status
-   :target: https://travis-ci.org/pytest-dev/pytest-django
 .. image:: https://img.shields.io/pypi/v/pytest-django.svg?style=flat
-   :alt: PyPI Version
-   :target: https://pypi.python.org/pypi/pytest-django
+    :alt: PyPI Version
+    :target: https://pypi.python.org/pypi/pytest-django
+
+.. image:: https://img.shields.io/pypi/pyversions/pytest-django.svg
+    :alt: Supported Python versions
+    :target: https://pypi.python.org/pypi/pytest-django
+
+.. image:: https://travis-ci.org/pytest-dev/pytest-django.svg?branch=master
+    :alt: Build Status
+    :target: https://travis-ci.org/pytest-dev/pytest-django
+
 .. image:: https://img.shields.io/codecov/c/github/pytest-dev/pytest-django.svg?style=flat
-   :alt: Coverage
-   :target: https://codecov.io/gh/pytest-dev/pytest-django
+    :alt: Coverage
+    :target: https://codecov.io/gh/pytest-dev/pytest-django
 
 Welcome to pytest-django!
 =========================