install python serial for travis

.travis.yml

@@ -10,6 +10,7 @@ os:
 script:
   #- set -e
   - echo -e "travis_fold:start:sketch_test_env_prepare"
+  - pip install serial
   - wget -O arduino.tar.xz https://www.arduino.cc/download.php?f=/arduino-nightly-linux64.tar.xz
   - tar xf arduino.tar.xz
   - mv arduino-nightly $HOME/arduino_ide