Changes

• Issue #216: update to Checkstyle 8.30
• Upgrade Maven enforcer plugin
• Use Tycho 1.6.0 and JDK 13 in tests
• Issue checkstyle/checkstyle#7444: disable google mirror in Travis
• Issue #210: keep .gitignore on clean phase

Please read the Checkstyle release notes for details on recent and potentially breaking changes in Checkstyle. Checkstyle configuration files potentially need to be adapted to this version.