By Nate Raymond BOSTON (Reuters) - Massachusetts regulators on Thursday sought the revocation of Robinhood's broker-dealer license after charging that it encourages inexperienced investors to place risky trades without limits, while the online brokerage sued to invalidate a new rule underlying the case. Massachusetts Secretary of State Bill Galvin sought the revocation in a revised administrative case announced shortly after Robinhood sued in state court in Boston to challenge a fiduciary standard of conduct his office adopted last year.