Merge pull request #39 from graste/sf4

min version php 7.2 + bump deps + add php7.4 in CI

fix psr2 issue

adjust test to symfony version bump

bump composer deps

add expression exception handling to VariableGuard

to give more information on what failed when symfony expression throws an
exception. We throw a Workflux Error and set the original exception as the
previous exception for stacktrace evaluation.

adjust travis build matrix

Remove unused code

Update CHANGELOG.md

adjust test to now quoted digraph name

update graste/params dependency version