untagged-7c884a9a38903197f4a2: Symfony 4 (#48)
* Reduce supported PHP versions to 7.1+, and support Symfony 4+ only. Use older versions for Symfony 2 and 3. * Document versions / branches * Update composer test scenarios. * Update lowest version constraints * More 'lowest' pinning * fromShellCommandline is no longer needed * Make function prototypes match base class more closely