tool/generic_erb.rb: Use ERB#result_with_hash instead of #result

to prevent the warnings:
```
./tool/generic_erb.rb:23: warning: assigned but unused variable - output
./tool/generic_erb.rb:24: warning: assigned but unused variable - vpath
```

tool/generic_erb.rb

@@ -30,7 +30,7 @@
     erb = ERB.new(File.read(template), nil, '%-')
   end
   erb.filename = template
-  source ? erb.src : proc{erb.result(binding)}.call
+  source ? erb.src : proc{erb.result_with_hash({ erb: erb, output: output, vpath: vpath })}.call
 end
 result = result.size == 1 ? result[0] : result.join("")
 out.write(result)