Cannot get PHPUnit xml file to detect tests with bin/phpunit command

composer.json

{
  "require": {
    "phpspec/phpspec": "2.0.*@dev",
    "phpunit/phpunit": "3.7.31",
    "behat/behat": "2.4.*@stable",
    "behat/mink": "1.5.0",
    "behat/mink-extension": "*",
    "behat/mink-goutte-driver": "*",
    "behat/mink-selenium-driver": "*",
    "behat/mink-selenium2-driver": "*",
    "raveren/kint": "dev-1.0.0-wip",
    "chriskite/phactory": "v0.4.3"
  },
  "require-dev": {
      "phpspec/phpspec": "2.0.*@dev"
  },
  "minimum-stability": "dev",
  "config": {
      "bin-dir": "bin/"
  },
  "autoload": {
    "psr-0": {
      "": "src"
    },
    "classmap": ["lib/"],
    "files": ["includes/credentials.php"]
  }
}

console output

$ bin/phpunit
PHPUnit 3.7.31 by Sebastian Bergmann.

Configuration read from /Path/To/Project/phpunit.xml



Time: 24 ms, Memory: 1.75Mb

No tests executed!

FooTest.php

# located in tests/FooTest.php

<?php

class FooTest extends PHPUnit_Framework_TestCase
{
    public function it_should_work()
    {
        $foo = true;
        $this->assertTrue($foo);
    }
}

phpunit.xml

<phpunit boostrap="vendor/autoload.php"
         colors="true"
         convertErrorsToException="true"
         convertNoticesToExceptions="true"
         convertWarningsToExceptions="true"
         processIsolation="false"
         stopOnFailure="false"
         syntaxCheck="false">
  <testsuites>
    <testsuite name="Tests">
      <directory suffix="Test.php">
        tests
      </directory>
    </testsuite>
  </testsuites>
</phpunit>