docs: update travis image url
**Open Source framework for large-scale digital repositories.**
-.. image:: https://img.shields.io/travis/inveniosoftware/invenio.svg
+.. image:: https://travis-ci.org/inveniosoftware/invenio.svg?branch=master
:target: https://travis-ci.org/inveniosoftware/invenio
.. image:: https://img.shields.io/coveralls/inveniosoftware/invenio.svg
