[travis] Run real php subprocesses on hhvm for Process component tests

.travis.yml

@@ -13,6 +13,7 @@ addons:
 env:
     global:
         - MIN_PHP=5.3.3
+        - SYMFONY_PROCESS_PHP_TEST_BINARY=~/.phpenv/versions/5.6/bin/php
 
 matrix:
     include:

src/Symfony/Component/Process/Tests/ProcessTest.php

@@ -30,7 +30,7 @@ class ProcessTest extends \PHPUnit_Framework_TestCase
     public static function setUpBeforeClass()
     {
         $phpBin = new PhpExecutableFinder();
-        self::$phpBin = 'phpdbg' === PHP_SAPI ? 'php' : $phpBin->find();
+        self::$phpBin = getenv('SYMFONY_PROCESS_PHP_TEST_BINARY') ?: ('phpdbg' === PHP_SAPI ? 'php' : $phpBin->find());
         if ('\\' !== DIRECTORY_SEPARATOR) {
             // exec is mandatory to deal with sending a signal to the process
             // see https://github.com/symfony/symfony/issues/5030 about prepending