It is hereby declared to be against public policy for any employer to employ any person in an occupation in this commonwealth at an oppressive and unreasonable wage as defined in section two, and any contract, agreement or understanding for or in relation to such employment shall be null and void. A wage of less than $15.00 per hour, in any occupation, as defined in this chapter, shall conclusively be presumed to be oppressive and unreasonable, wherever the term "minimum wage" is used in this chapter