[#989]: Update monolog requirement to 1.6

phpDocumentor now uses the ErrorHandler of Monolog but that wasn't available
until 1.6. Given that the composer.json is locked on 1.4 we need to bump the
requirement to 1.6.
Merge pull request #990 from mvriel/989

[#989]: Update monolog requirement to 1.6
Stable version constraints for all dependencies

Also update dependency specs in composer.lock