Also increase AppVeyor cache size to 1024,
should match what is available for us in the free plan:
https://www.appveyor.com/docs/build-cache/#cache-size-beta
And drop obsolete debug_release option for Windows, superseded
by target=release and debug_symbols=yes.
Now that AppVeyor handles testing Windows builds with MSVC,
we can skip the need to test against Travis' old MinGW toolchain
and have a faster CI process.
Also try building X11 binaries against system OpenSSL, should
speed up build.