Use env for Arduino version

Author: FokkeZB

.travis.yml

@@ -1,11 +1,13 @@
 language: c
+env:
+  - ARDUINO_VERSION=1.6.13
 before_install:
   - "/sbin/start-stop-daemon --start --quiet --pidfile /tmp/custom_xvfb_1.pid --make-pidfile --background --exec /usr/bin/Xvfb -- :1 -ac -screen 0 1280x1024x16"
   - sleep 3
   - export DISPLAY=:1.0
-  - wget http://downloads.arduino.cc/arduino-1.6.13-linux64.tar.xz
-  - tar xf arduino-1.6.13-linux64.tar.xz
-  - sudo mv arduino-1.6.13 /usr/local/share/arduino
+  - wget http://downloads.arduino.cc/arduino-$ARDUINO_VERSION-linux64.tar.xz
+  - tar xf arduino-$ARDUINO_VERSION-linux64.tar.xz
+  - sudo mv arduino-$ARDUINO_VERSION /usr/local/share/arduino
   - sudo ln -s /usr/local/share/arduino/arduino /usr/local/bin/arduino
 install:
   - ln -s $PWD /usr/local/share/arduino/libraries/TheThingsNetwork