Configure Rails/UnknownEnv rubocop rule

We use staging and preproduction environments, which are not valid by
default.

This rule is useful because misspelling the name of an environment might
otherwise go unnoticed.

.rubocop.yml

@@ -165,9 +165,6 @@ Rails/SkipsModelValidations:
 Rails/UniqBeforePluck:
   Enabled: true
 
-Rails/UnknownEnv:
-  Enabled: true
-
 Rails/Validation:
   Enabled: true
 

.rubocop_basic.yml

@@ -148,6 +148,15 @@ Rails/SafeNavigation:
 Rails/TimeZone:
   Enabled: true
 
+Rails/UnknownEnv:
+  Enabled: true
+  Environments:
+    - development
+    - test
+    - production
+    - preproduction
+    - staging
+
 RSpec/NotToNot:
   Enabled: true