Track release of new Mysqldump-php and update composer.json

[bomoko]

This issue is a reminder/placeholder for future work.
Presently, we need to work with the dev-master version of mysqldump-php in order to incorporate one of the override hooks.

As soon as the new version of 2.4 (or greater) is released, we need to lock out the composer.json reference to that.

[back-2-95]

PR #50 will bump the constraint to latest 2.9