.travis.yml: Explicitly install python-serial for ESP32 builds.
.travis.yml: Explicitly install python-serial for ESP32 builds.

It's dependency of esptool.py, and don't rely on being preinstalled.

Change-Id: Ie4f8e40a0669d54273294b64c90ad8aafd2b4996
Signed-off-by: Paul Sokolovsky <9dc5061f178bb11813e14f1ed02ba27eaf21fa5d@users.sourceforge.net>

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython