docs: remove coverage badge
* Removes test coverage badge because it is misleading since Invenio
is a container package and hence almost have no code.
.. 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
- :target: https://coveralls.io/r/inveniosoftware/invenio
-
.. image:: https://badges.gitter.im/Join%20Chat.svg
:target: https://gitter.im/inveniosoftware/invenio
