require recent whoosh
    
    Olders one throws traceback when editing form in FE.
    
    Exception on /coprs/detail/msuchy/copr/update/ [POST]
    Traceback (most recent call last):
      File "/usr/lib/python2.7/site-packages/flask/app.py", line 1687, in wsgi_app
        response = self.full_dispatch_request()
      File "/usr/lib/python2.7/site-packages/flask/app.py", line 1360, in full_dispatch_request
        rv = self.handle_user_exception(e)
      File "/usr/lib/python2.7/site-packages/flask/app.py", line 1358, in full_dispatch_request
        rv = self.dispatch_request()
      File "/usr/lib/python2.7/site-packages/flask/app.py", line 1344, in dispatch_request
        return self.view_functions[rule.endpoint](**req.view_args)
      File "/usr/share/copr/coprs_frontend/coprs/views/misc.py", line 120, in decorated_function
        return f(*args, **kwargs)
      File "/usr/share/copr/coprs_frontend/coprs/views/coprs_ns/coprs_general.py", line 185, in copr_update
        db.session.commit()
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 149, in do
        return getattr(self.registry(), name)(*args, **kwargs)
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 721, in commit
        self.transaction.commit()
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/orm/session.py", line 361, in commit
        self.session.dispatch.after_commit(self.session)
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/event.py", line 409, in __call__
        fn(*args, **kw)
      File "/usr/lib/python2.7/site-packages/flask_sqlalchemy.py", line 185, in after_commit
        models_committed.send(session.app, changes=d.values())
      File "/usr/lib/python2.7/site-packages/blinker/base.py", line 220, in send
        for receiver in self.receivers_for(sender)]
      File "/usr/lib/python2.7/site-packages/flask_whooshee.py", line 179, in on_commit
        writer.commit()
      File "/usr/lib/python2.7/site-packages/whoosh/writing.py", line 907, in commit
        self._commit_toc(finalsegments)
      File "/usr/lib/python2.7/site-packages/whoosh/writing.py", line 854, in _commit_toc
        toc.write(self.storage, self.indexname)
      File "/usr/lib/python2.7/site-packages/whoosh/index.py", line 688, in write
        stream.write_string(pickle.dumps(schema, -1))
      File "/usr/lib/python2.7/site-packages/whoosh/fields.py", line 502, in __getstate__
        del d["_struct"]
    KeyError: '_struct'