Add reject stub for credentials testing

Upstream in `fog-core` the method was inverted to use `select` rather
than a negated reject for Rubocop style rules. That caused this spec to
fail as only `reject` was stubbed.

This stubs `select` as well so this should work for both versions, older
versions of `fog-core` and newer ones.

spec/fog/compute/brightbox_spec.rb

@@ -14,10 +14,16 @@
       }
 
       @credential_guard = Minitest::Mock.new
+      # Stub for older versions of fog-core
       def @credential_guard.reject
         {}
       end
 
+      # Stub for newer versions of fog-core
+      def @credential_guard.select
+        {}
+      end
+
       Fog.stub :credentials, @credential_guard do
         @service = Fog::Brightbox::Compute.new(@arguments)
       end