Allow using custom binary for phpunit (instead of default 4.X from phpqa/vendor/bin/phpunit)

[zdenekdrahos]

1. phpqa is installed globally
2. phpqa's phpunit version != project's phpunit, e.g. v4.8.21 vs v5.7.19

phpqa --analyzedDirs src --tools phpunit:0 --verbose
 [FileSystem\CleanDir] Cleaned build/
 [FileSystem\FilesystemStack] mkdir ["build\/"]
 [Edge\QA\Task\ParallelExec] "/qa/vendor/bin/phpunit" --configuration=phpunit.xml --log-junit="build//log-junit.xml" --log-tap="build//log-tap.text" --testdox-html="build//testdox-html.html" --testdox-text="build//testdox-text.txt"
 1/1 [============================] 100%
 [Edge\QA\Task\ParallelExec] Output for  "/qa/vendor/bin/phpunit" --configuration=phpunit.xml --log-junit="build//log-junit.xml" --log-tap="build//log-tap.text" --testdox-html="build//testdox-html.html" --testdox-text="build//testdox-text.txt"

Fatal error: Uncaught Error: Call to undefined method PHPUnit_Util_Configuration::getTestdoxGroupConfiguration() in /my-project/vendor/phpunit/phpunit/src/TextUI/TestRunner.php:1066
Stack trace:
#0 /my-project/vendor/phpunit/phpunit/src/TextUI/TestRunner.php(163): PHPUnit_TextUI_TestRunner->handleConfiguration(Array)
#1 /qa/vendor/phpunit/phpunit/src/TextUI/Command.php(149): PHPUnit_TextUI_TestRunner->doRun(Object(PHPUnit_Framework_TestSuite), Array)
#2 /qa/vendor/phpunit/phpunit/src/TextUI/Command.php(100): PHPUnit_TextUI_Command->run(Array, true)
#3 /qa/vendor/phpunit/phpunit/phpunit(47): PHPUnit_TextUI_Command::main()
#4 {main}
  thrown in /my-project/vendor/phpunit/phpunit/src/TextUI/TestRunner.php on line 1066

Everything is fine when /my-project/vendor/bin/phpunit is used