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ARROW-5448: [C++][CI][MinGW] Ensure updating GCC and Python [skip travis]

"import numpy" is failed because GCC and Python are older than GCC and Python that are used for building numpy binary packages.
We can run "import numpy" successfully by upgrading GCC and Python to the latest versions.

Author: Sutou Kouhei <kou@clear-code.com>

Closes #4428 from kou/glib-appveyor and squashes the following commits:

15fb6670 <Sutou Kouhei>  Ensure updating GCC and Python