message: [bugfix] remove quotes from Postgres time grains
diff: @@ -216,14 +216,14 @@ class PostgresEngineSpec(BaseEngineSpec):
  time_grains = (
  Grain('Time Column', _('Time Column'), '{col}'),
  - Grain('second', _('second'), "DATE_TRUNC('second', '{col}')"),
  - Grain('minute', _('minute'), "DATE_TRUNC('minute', '{col}')"),
  - Grain('hour', _('hour'), "DATE_TRUNC('hour', '{col}')"),
  - Grain('day', _('day'), "DATE_TRUNC('day', '{col}')"),
  - Grain('week', _('week'), "DATE_TRUNC('week', '{col}')"),
  - Grain('month', _('month'), "DATE_TRUNC('month', '{col}')"),
  - Grain('quarter', _('quarter'), "DATE_TRUNC('quarter', '{col}')"),
  - Grain('year', _('year'), "DATE_TRUNC('year', '{col}')"),
  + Grain('second', _('second'), "DATE_TRUNC('second', {col})"),
  + Grain('minute', _('minute'), "DATE_TRUNC('minute', {col})"),
  + Grain('hour', _('hour'), "DATE_TRUNC('hour', {col})"),
  + Grain('day', _('day'), "DATE_TRUNC('day', {col})"),
  + Grain('week', _('week'), "DATE_TRUNC('week', {col})"),
  + Grain('month', _('month'), "DATE_TRUNC('month', {col})"),
  + Grain('quarter', _('quarter'), "DATE_TRUNC('quarter', {col})"),
  + Grain('year', _('year'), "DATE_TRUNC('year', {col})"),
  )
  @classmethod
  