Use Environment to obtain PHP binary

This improves support for HHVM by ensuring we use the HHVM binary for child processes and invoke it with the "--php" command line option.

Author: jmikola

composer.json

@@ -9,6 +9,9 @@
         "react/event-loop": "0.3.*|0.4.*",
         "react/stream": "0.3.*|0.4.*"
     },
+    "require-dev": {
+        "sebastian/environment": "~1.0"
+    },
     "autoload": {
         "psr-4": { "React\\ChildProcess\\": "src" }
     },

tests/AbstractProcessTest.php

@@ -4,6 +4,7 @@
 
 use React\ChildProcess\Process;
 use React\EventLoop\Timer\Timer;
+use SebastianBergmann\Environment\Runtime;
 
 abstract class AbstractProcessTest extends \PHPUnit_Framework_TestCase
 {
@@ -329,6 +330,8 @@ public function assertSoon(\Closure $callback, $timeout = 20000, $interval = 200
 
     private function getPhpBinary()
     {
-        return defined('PHP_BINARY') ? PHP_BINARY : 'php';
+        $runtime = new Runtime();
+
+        return $runtime->getBinary();
     }
 }