Older version of composer used by Travis-CI accepted a dependency failure for PHP<7.1 and Symfony 4, but new composer version explicitly fails to install a dependency, causing those builds to fail.
This PR explicitly declares which builds to conduct in the matrix with known good combinations, and extends to include PHP 7.3.
Older version of composer used by Travis-CI accepted a dependency failure for PHP<7.1 and Symfony 4, but new composer version explicitly fails to install a dependency, causing those builds to fail.
This PR explicitly declares which builds to conduct in the matrix with known good combinations, and extends to include PHP 7.3.