Enable useless assignment rubocop rule

This way deverlopers who don't run the ruby syntax check locally with
warnings enabled will be informed by HoundCI.

.rubocop_basic.yml

@@ -30,6 +30,9 @@ Layout/TrailingBlankLines:
 Layout/TrailingWhitespace:
   Enabled: true
 
+Lint/UselessAssignment:
+  Enabled: true
+
 Metrics/LineLength:
   Max: 100