Mark ext-ast as suggestion, not a requirement.

The point of this library is to make it possible for users
to run code without building ext-ast.

- Only needed for unit tests

Author: TysonAndre

composer.json

@@ -6,7 +6,9 @@
         "nikic/PHP-Parser": "3.0.5"
     },
     "require-dev": {
-        "phpunit/phpunit": "~6.2",
+        "phpunit/phpunit": "~6.2"
+    },
+    "suggest": {
         "ext-ast": "~0.1.4"
     },
     "autoload": {