travis: allow_failures for 7.3 because not supported (friendsofphp/php-cs-fixer v2.9.3 requires php ^5.6 || >=7.0 <7.3)
travis: allow_failures for 7.3 because not supported (friendsofphp/php-cs-fixer v2.9.3 requires php ^5.6 || >=7.0 <7.3)

Fahl-Design/log4php-slack-appender,Fahl-Design/log4php-slack-appender