title: how to get the last day of the month from a given date
body: <p>I have a column of dates in my dataframe and I'd like to get the last day of the month from the dates
  example, if the date is '2017-01-25' I want to get '2017-01-31'
  I suppose I can get the month and year number from the dates and use monthrange to figure out the last day of the month but I'm looking for a one line code  </p>