Added Arduino 1.6.8 to the continuous integration

.travis.yml

@@ -74,6 +74,7 @@ matrix:
       env: SCRIPT=coverage
     - env: SCRIPT=arduino VERSION=1.5.8 BOARD=arduino:avr:uno
     - env: SCRIPT=arduino VERSION=1.6.7 BOARD=arduino:avr:uno
+    - env: SCRIPT=arduino VERSION=1.6.8 BOARD=arduino:avr:uno
     - env: SCRIPT=platformio BOARD=uno
     - env: SCRIPT=platformio BOARD=due
     - env: SCRIPT=platformio BOARD=esp01