Checkstyle Sonar plugin 4.21

Checkstyle Sonar plugin 4.21 - https://github.com/checkstyle/sonar-checkstyle/releases/tag/4.21

Changes:
Issue #222: update to CS 8.21
Issue #225: resolve IDEA violation
ci: Travis can not substitute variable in such format
ci: update travis.sh to use argument instead of TEST variable