ci: Travis: ccache: use --zero-stats

This makes it display actual results in before_cache then, and it seems
like it is not necessary to keep stats across builds.

ci/before_script.sh

@@ -20,8 +20,8 @@ if [[ -n "${LLVM_SYMBOLIZER}" ]] && [[ ! $(type -P "${LLVM_SYMBOLIZER}") ]]; the
   exit 1
 fi
 
-# Show ccache stats so we can compare in before_cache
-ccache -s 2>/dev/null || true
+# Reset ccache stats for real results in before_cache.
+ccache --zero-stats
 
 if [[ "${TRAVIS_OS_NAME}" == osx ]]; then
   # Adds user to a dummy group.