Fix the CI on Appveyor

On windows, composer does not use a symlink when installing binaries but
creates a proxy script instead. This proxy is not a PHP script, so running it
through PHP does not work.
The goal of composer is to allow running the script in a way similar to the
Linux usage relying on shebangs.

Author: stof

appveyor.yml

@@ -48,4 +48,4 @@ install:
 
 test_script:
     - cd c:\projects\symfony-demo
-    - php vendor/bin/simple-phpunit
+    - vendor/bin/simple-phpunit