(MAINT) Add Exclude setting

When inheriting from a gem, the AllCops/exclude property doesn't seem to
be propagated.

Running rubocop with --degbug confirms that it is recognising the
directive to inherit .rubocop.yml from puppet-line and rules seem to be
valid.

This commit adds the AllCops/Exclude containing a single value that
excludes the "vendor" directory that appears when running in ci.

.rubocop.yml

@@ -1,2 +1,6 @@
 inherit_gem:
-  puppet-lint: .rubocop.yml
+  puppet-lint:
+    - .rubocop.yml
+AllCops:
+   Exclude:
+   - "vendor/**/*"