Corrected wrench config for `src/site` merges

Even though the gulp install script outputs:
```Site folder exists, merging files (no overwrite) project-dir/semantic/src/site/```

The `src/site` directory is blown away, not preserving existing files, due to the `forceDelete:true` setting.

tasks/config/project/install.js

@@ -752,11 +752,11 @@ module.exports = {
 
       // copy for site theme
       site: {
-        forceDelete       : true,
+        forceDelete       : false,
         excludeHiddenUnix : true,
         preserveFiles     : true
       }
     }
   }
 
-};
+};