travis.yml: Running tests in parallel on travis is slower
travis.yml: Running tests in parallel on travis is slower

.. so explicitly set them to run serially with -j1.

(as discussed on #729)

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/micropython,adafruit/micropython,adafruit/circuitpython,adafruit/circuitpython,adafruit/micropython,adafruit/circuitpython,adafruit/micropython