Section 179A. In the awarding of contracts for public work by the commonwealth or by a county, city or town or by persons contracting therewith to do such work, preference shall be given to persons who are citizens of the United States and to partnerships all of whose members are such citizens. Any person who knowingly and wilfully violates this section shall be punished by a fine of not more than two hundred dollars. Nothing in this section shall require the acceptance of a higher bid in preference to a lower bid.