travis: Enable caching for sonarcloud scan

scripts/test.sh

@@ -433,6 +433,8 @@ sonarcloud)
 		-Dsonar.projectVersion=$(git describe)+${TRAVIS_BUILD_NUMBER} \
 		-Dsonar.sources=. \
 		-Dsonar.cfamily.threads=2 \
+		-Dsonar.cfamily.cache.enabled=true \
+		-Dsonar.cfamily.cache.path=$HOME/.sonar-cache \
 		-Dsonar.cfamily.build-wrapper-output=bw-output || exit $?
 	rm -r bw-output .scannerwork
 	;;