docs: add travis, pypi and license badges
**Open Source framework for large-scale digital repositories.**
+.. image:: https://img.shields.io/pypi/v/invenio.svg
+ :target: https://pypi.org/project/invenio/
+
+.. image:: https://img.shields.io/github/license/inveniosoftware/invenio.svg
+ :target: https://github.com/inveniosoftware/invenio/blob/master/LICENSE
+
.. image:: https://travis-ci.org/inveniosoftware/invenio.svg?branch=master
:target: https://travis-ci.org/inveniosoftware/invenio
