Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

core/modules/contact/config/install/contact.form.personal.yml

@@ -1,7 +1,8 @@
+langcode: en
+status: true
+dependencies: {  }
 id: personal
 label: 'Personal contact form'
 recipients: {  }
 reply: ''
 weight: 0
-status: true
-langcode: en

core/modules/contact/config/schema/contact.schema.yml

@@ -43,14 +43,3 @@ contact.settings:
     user_default_enabled:
       type: boolean
       label: 'Personal contact form enabled by default'
-
-migrate.source.d6_contact_settings:
-  type: migrate_source_sql
-  label: 'Drupal 6 contact settings'
-  mapping:
-    variables:
-      type: sequence
-      label: 'Variables'
-      sequence:
-        type: string
-        label: 'Variable'

core/modules/contact/config/schema/contact.source.schema.yml

@@ -0,0 +1,12 @@
+# Schema for the migration source plugins.
+
+migrate.source.contact_settings:
+  type: migrate_source_sql
+  label: 'Drupal contact settings'
+  mapping:
+    variables:
+      type: sequence
+      label: 'Variables'
+      sequence:
+        type: string
+        label: 'Variable'