Setting CleanWebpackPlugin's cleanStaleWebpackAssets: false

There is currently a bug where, on a "watch" rebuild, sometimes
an image that's referenced from a CSS file won't be "noticed" and
will be mistakenly deleted.

Author: weaverryan

lib/plugins/clean.js

@@ -29,7 +29,9 @@ module.exports = function(plugins, webpackConfig) {
 
     const cleanWebpackPluginOptions = {
         verbose: false,
-        cleanOnceBeforeBuildPatterns: webpackConfig.cleanWebpackPluginPaths
+        cleanOnceBeforeBuildPatterns: webpackConfig.cleanWebpackPluginPaths,
+        // disabled to avoid a bug where some files were incorrectly deleted on watch rebuild
+        cleanStaleWebpackAssets: false
     };
 
     plugins.push({