windows: Make appveyor.yml self-contained.
windows: Make appveyor.yml self-contained.

Add configuration which otherwise has to be set via the UI so the file is
more self-contained, and remove configuration which is not needed because
it's the same as the default.  The major change here is that for a while
now Appveyor has been using Visual Studio 2015 by default while we still
want to support 2013.

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