installation: python-twitter requirement update
installation: python-twitter requirement update

* Updates python-twitter in requirements.txt to the latest version
  (2.0) that supports Twitter 1.1 API.  (PR #2015)

Signed-off-by: Sebastian Witowski <f69e5de8dea812f55abb37bf79df36126074c5c0@cern.ch>
Tested-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

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