rubocop.yml: exclude fixtures directory

When we've a fixtures directory we should not lint the fixtures. Those
are usually files that we us in our tests to compare output of certain
things.

rubocop.yml

@@ -14,6 +14,7 @@ AllCops:
   Exclude:
     - vendor/**/*
     - .vendor/**/*
+    - spec/fixtures/**/*
 
 # this currently doesn't work with the way we handle our secrets
 Gemspec/RequireMFA: