about HF20 curation reward penalty changed from 30 minutes to 15minutes

[wjashan]

    def curation_reward_pct(self):
        """ If post is less than 30 minutes old, it will incur a curation
        reward penalty.  """
        reward = (self.time_elapsed().seconds / 1800) * 100
        if reward > 100:
            reward = 100
        return reward

change to:

    def curation_reward_pct(self):
        """ If post is less than 15 minutes old, it will incur a curation
        reward penalty.  """
        reward = (self.time_elapsed().seconds / 900) * 100
        if reward > 100:
            reward = 100
        return reward