Replace app/bootstrap wildcard with explicit entry

See https://github.com/symfony/symfony-standard/commit/e7b7b0f366465b15a29f44d7ba17287bb0b8392b

Symfony2.gitignore

@@ -1,5 +1,5 @@
 # Bootstrap
-app/bootstrap*
+app/bootstrap.php.cache
 
 # Symfony directories
 vendor/*