docs: fix installation
docs: fix installation

* Uses the pip1.6 ready way of installation, no dependency links.

Signed-off-by: Yoan Blanc <7e8c88beed64bd6a341c3085bc82ac100e832f6e@cern.ch>

inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio