appveyor: Build with MSVC 2015.

The MSVC version we (at VMware) primarily care about from now on is
2015.

See https://ci.appveyor.com/project/jrfonseca/mesa/build/46

We can drop support for building with 2013 in a future commit.  I'm not
aware of significant changes in C99/C11 support from MSVC 2013 to 2015,
but there's no point in continuing supporting old MSVC versions when
nobody cares.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
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