Remove `pymysql` related warnings

[eywalker]

Currently, when you use DJ, you are met with a lengthy warning from PyMySQL

/usr/local/lib/python3.5/dist-packages/pymysql/cursors.py:165: Warning: (1681, "'ERROR_FOR_DIVISION_BY_ZERO' is deprecated and will be removed in a future release.")
  result = self._query(query)
/usr/local/lib/python3.5/dist-packages/pymysql/cursors.py:165: Warning: (1681, "'NO_ZERO_DATE' is deprecated and will be removed in a future release.")
  result = self._query(query)
/usr/local/lib/python3.5/dist-packages/pymysql/cursors.py:165: Warning: (1681, "'NO_ZERO_IN_DATE' is deprecated and will be removed in a future release.")
  result = self._query(query)

This appears to be triggered by MySQL returning warnings for these mode flags. These warning flags should be suppressed.