- Fixing CMake to always force python3 binary (this should always work)
- Adding macOS Travis build (clang and gcc)
- Disabling all clang builds until macOS (lack of packages) and Linux (broken Travis repos) are fixed

clang builds are currently failing (travis dependency issue)

- Adds the following tests
  * gcc + clang
    + IC60
    + IC60_LED
    + ICED
    + WhiteFox
    + Testing/uartOut
    + Testing/usbMuxUart
  * gcc
    + Bootloader/mk20dx128vlf5
    + Bootloader/mk20dx256vlh7