travis: Build unix nanbox with PYTHON=python2.
travis: Build unix nanbox with PYTHON=python2.

To test build support with Python 2.7.

bvernoux/micropython,tobbad/micropython,pramasoul/micropython,bvernoux/micropython,trezor/micropython,henriknelson/micropython,selste/micropython,selste/micropython,bvernoux/micropython,pramasoul/micropython,adafruit/circuitpython,pozetroninc/micropython,bvernoux/micropython,trezor/micropython,kerneltask/micropython,pramasoul/micropython,henriknelson/micropython,kerneltask/micropython,adafruit/circuitpython,henriknelson/micropython,kerneltask/micropython,pramasoul/micropython,MrSurly/micropython,adafruit/circuitpython,trezor/micropython,pozetroninc/micropython,selste/micropython,henriknelson/micropython,adafruit/circuitpython,trezor/micropython,tobbad/micropython,MrSurly/micropython,pramasoul/micropython,adafruit/circuitpython,pozetroninc/micropython,MrSurly/micropython,tobbad/micropython,kerneltask/micropython,trezor/micropython,adafruit/circuitpython,bvernoux/micropython,selste/micropython,tobbad/micropython,pozetroninc/micropython,tobbad/micropython,MrSurly/micropython,selste/micropython,kerneltask/micropython,MrSurly/micropython,pozetroninc/micropython,henriknelson/micropython