Switch tools declared in composer.json from system to Composer version.
That fixes compatibility issues, and .phar packaging issues.
Run correctly phpunit so build.xml can also be used in local,
not only on Jenkins loading dynamically XDebug.
Test Plan: ant
Reviewed By: DorianWinty
Maniphest Tasks: T1731
Differential Revision: https://devcentral.nasqueron.org/D2683