checksum code really workin now

fixes https://github.com/adafruit/Adafruit-GPS-Library/issues/40

Adafruit_GPS.cpp

@@ -39,12 +39,12 @@ boolean Adafruit_GPS::parse(char *nmea) {
     sum += parseHex(nmea[strlen(nmea)-2]);
     
     // check checksum 
-    for (uint8_t i=1; i < (strlen(nmea)-4); i++) {
+    for (uint8_t i=2; i < (strlen(nmea)-4); i++) {
       sum ^= nmea[i];
     }
     if (sum != 0) {
       // bad checksum :(
-      //return false;
+      return false;
     }
   }
   int32_t degree;