Update adafruit_mcp9808.py (#5)

This change corrects an error in the temperature function that causes it not to return negative values for the temperature when below 0 degrees Celsius.

Fixes #4

Author: ryespy

adafruit_mcp9808.py

@@ -96,7 +96,7 @@ def temperature(self):
         self.buf[1] = self.buf[1] & 0x1f
         if self.buf[1] & 0x10 == 0x10:
             self.buf[1] = self.buf[1] & 0x0f
-            return 256 - (self.buf[1] * 16 + self.buf[2] / 16.0)
+            return (self.buf[1] * 16 + self.buf[2] / 16.0) - 256
         else:
             return self.buf[1] * 16 + self.buf[2] / 16.0