travis: Decrease build duration by starting OSX build early.
travis: Decrease build duration by starting OSX build early.

Looking at the recent build history the time it takes just to complete the
OSX build is already 12 minutes so make it start early, which brings down
the total build time from about 20 minutes to 14 minutes.

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