Correct initialisation of next_update

For 'services' without catchup (e.g. mqtt) this was causing no data to
be sent when testing by running pywws.toservice from the command line.

Signed-off-by: Jim Easterbrook <jim@jim-easterbrook.me.uk>

src/pywws/__init__.py

@@ -1,3 +1,3 @@
-__version__ = '17.07.0'
-_release = '1376'
-_commit = '3827aa8'
+__version__ = '17.11.0'
+_release = '1377'
+_commit = '6489c63'

src/pywws/toservice.py

@@ -281,6 +281,7 @@ def __init__(self, params, status, calib_data, service_name):
         # set timestamp of first data to upload
         self.next_update = datetime.utcnow() - max(
             timedelta(days=self.catchup), self.interval)
+        self.next_update = min(self.next_update, self.data.before(datetime.max))
 
     def prepare_data(self, data):
         """Prepare a weather data record.