fix first read issue

Author: jerryneedell

adafruit_dht.py

@@ -138,7 +138,7 @@ def measure(self):
         delay_between_readings = 0.5
         if self._dht11:
             delay_between_readings = 1.0
-        if time.monotonic()-self._last_called > delay_between_readings:
+        if (self._last_called == 0) or (time.monotonic()-self._last_called > delay_between_readings):
             self._last_called = time.monotonic()
 
             pulses = self._get_pulses()