Add default parameter packagist_host, fixes #412

app/config/config.yml

@@ -1,4 +1,5 @@
 imports:
+    - { resource: defaults.yml }
     - { resource: parameters.yml }
     - { resource: security.yml }
 

app/config/defaults.yml

@@ -0,0 +1,2 @@
+parameters:
+    packagist_host: ~