[fix] Update multi-instance and requirements in the manifest

manifest.json

@@ -12,7 +12,10 @@
         "email": "john.doe@example.com",
         "url": "http://example.com"
     },
-    "multi_instance": "true",
+    "requirements": {
+        "yunohost": ">> 2.3.12.1"
+    },
+    "multi_instance": true,
     "services": [
         "nginx",
         "php5-fpm",