Merge pull request firmata#192 from mgcrea/master

feat(boards): add support for teensy 3.1

Author: soundanalogous

Boards.h

@@ -250,8 +250,8 @@ writePort(port, value, bitmask):  Write an 8 bit port.
 #define PIN_TO_SERVO(p)         (p)
 
 
-// Teensy 3.0
-#elif defined(__MK20DX128__)
+// Teensy 3.0 and 3.1
+#elif defined(__MK20DX128__) || defined(__MK20DX256__)
 #define TOTAL_ANALOG_PINS       14
 #define TOTAL_PINS              38 // 24 digital + 10 analog-digital + 4 analog
 #define VERSION_BLINK_PIN       13