Modernize build.xml

Summary:
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.

Ref T1731.

Test Plan: ant

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1731

Differential Revision: https://devcentral.nasqueron.org/D2683