gh-620: Explicitly include InstalledVersions.php and `installed.php…

…` in PHAR

The autoloadmap includes a mapping `Composer\InstalledVersions` but as
of explicitly including `src` this file is no longer included.

This commit explicitly adds both `vendor/composer/InstalledVersions.php`
and `vendor/composer/installed.php` to ensure that the previous behavior
is preserved and WARNINGS are not issued when other packages use
`InstalledVersions::getVersion(// ...)`.

box.json

@@ -9,7 +9,9 @@
     "src/"
   ],
   "files": [
-    "manifest.xml"
+    "manifest.xml",
+    "vendor/composer/InstalledVersions.php",
+    "vendor/composer/installed.php"
   ],
   "git": "git",
   "main": "phar/phpunit-slow-test-detector.php",