travis: unittest2 requirements fix
travis: unittest2 requirements fix

* Fixes TypeError error on markupsafe library. (closes #2862)

Signed-off-by: Leonardo Rossi <5460ec99b71a72e7693c4088b5c2585e76e1c857@cern.ch>

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