Temporary use fysetc_f6_13 board for FYSETC F6 1.4

Until upstream PlatformIO board support is merged,
it is safe to use v1.3 board definition.

Reference: platformio/platform-atmelavr#180

platformio.ini

@@ -518,7 +518,8 @@ board    = fysetc_f6_13
 [env:FYSETC_F6_14]
 platform = atmelavr
 extends  = common_avr8
-board    = fysetc_f6_14
+# Workaround, until https://github.com/platformio/platform-atmelavr/pull/180 is merged
+board    = fysetc_f6_13
 
 #
 # Sanguinololu (ATmega644p)