ccache on Travis was never configured properly so the setting never
actually did anything. Enabling ccache for real brings other issues:
due to the way the preprocessor is handled, some of the Python header
macros produce bogus compiler warnings (which in turn produce errors
with -Werror). ccache also requires additional configuration on OS X
and docker. It would reduce compile time by ~30 seconds at best, so
it's not worth the trouble.

[skip appveyor]