Otherwise we run into situations where commits to stable branches
induce very long build times, as they have to basically build from
scratch but also invalidate the cache for future commits on the
master branch.
This commit also makes the cache folder branch-specific, but since
it's still limited to 1 GB of total cache size, we don't enable it
for non-master, as we would still run into issues with non-master
build invalidating the master cache.
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And drop obsolete debug_release option for Windows, superseded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.
