Update badges in README.rst

[lnielsen]

The badges in README.rst needs to be updated:

• Leave travis build status and coveralls test coverage
• Replace github tag with pypi release
• Remove pypi downloads (not available anymore) and license (showing unknown because of CERN waiver in the bottom of MIT license).

Example from Invenio-Indexer.

From:

.. image:: https://img.shields.io/travis/inveniosoftware/invenio-indexer.svg
        :target: https://travis-ci.org/inveniosoftware/invenio-indexer

.. image:: https://img.shields.io/coveralls/inveniosoftware/invenio-indexer.svg
        :target: https://coveralls.io/r/inveniosoftware/invenio-indexer

.. image:: https://img.shields.io/github/tag/inveniosoftware/invenio-indexer.svg
        :target: https://github.com/inveniosoftware/invenio-indexer/releases

.. image:: https://img.shields.io/pypi/dm/invenio-indexer.svg
        :target: https://pypi.python.org/pypi/invenio-indexer

.. image:: https://img.shields.io/github/license/inveniosoftware/invenio-indexer.svg
        :target: https://github.com/inveniosoftware/invenio-indexer/blob/master/LICENSE

To:

.. image:: https://img.shields.io/travis/inveniosoftware/invenio-indexer.svg
        :target: https://travis-ci.org/inveniosoftware/invenio-indexer

.. image:: https://img.shields.io/coveralls/inveniosoftware/invenio-indexer.svg
        :target: https://coveralls.io/r/inveniosoftware/invenio-indexer

.. image:: https://img.shields.io/pypi/v/invenio-indexer.svg
        :target: https://pypi.org/pypi/invenio-indexer

[krzysztof]

Done