Use /usr/bin/env instead of /bin/env.

bin/composer

@@ -1,4 +1,4 @@
-#!/bin/env php
+#!/usr/bin/env php
 <?php
 
 require __DIR__.'/../tests/bootstrap.php';
@@ -13,4 +13,4 @@ $composer->addDownloader('git', new GitDownloader);
 $composer->addInstaller('library', new LibraryInstaller);
 
 $cmd = new InstallCommand();
-$cmd->install($composer);
+$cmd->install($composer);