travis: Abandon mingw32 in favour of mingw-w64

This is actually long overdue: the README in the windows directory has been
updated once to indicate mingw32 is abandoned and not ok to use with uPy,
but we forgot travis builds were still using it.
As a bonus the travis build will succeed again since moduerrno.c now compiles.
(see https://github.com/micropython/micropython/pull/2399)

.travis.yml

@@ -12,7 +12,7 @@ before_script:
   - sudo add-apt-repository -y ppa:terry.guo/gcc-arm-embedded
   - sudo dpkg --add-architecture i386
   - sudo apt-get update -qq || true
-  - sudo apt-get install -y python3 gcc-multilib pkg-config libffi-dev libffi-dev:i386 qemu-system mingw32
+  - sudo apt-get install -y python3 gcc-multilib pkg-config libffi-dev libffi-dev:i386 qemu-system gcc-mingw-w64
   - sudo apt-get install -y --force-yes gcc-arm-none-eabi
   # For teensy build
   - sudo apt-get install realpath
@@ -36,7 +36,7 @@ script:
   - make -C teensy
   - make -C cc3200 BTARGET=application BTYPE=release
   - make -C cc3200 BTARGET=bootloader  BTYPE=release
-  - make -C windows CROSS_COMPILE=i586-mingw32msvc-
+  - make -C windows CROSS_COMPILE=i686-w64-mingw32-
 
   # run tests without coverage info
   #- (cd tests && MICROPY_CPYTHON3=python3.4 ./run-tests)