rake webpack: wrap execution in clean env.

This is needed, becasue the builder app has a separate Gemfile.

https://stackoverflow.com/a/31150424/2066546

#343 (comment)

#343 (comment)

Author: fiedl

Rakefile

@@ -40,15 +40,21 @@ namespace :webpack do
 
   namespace :build do
     task :development => 'yarn:install' do
-      sh "cd builder && bin/webpack"
+      Bundler.with_clean_env do
+        sh "cd builder && bin/webpack"
+      end
     end
     task :production => 'yarn:install' do
-      sh "cd builder && rake webpacker:compile"
+      Bundler.with_clean_env do
+        sh "cd builder && bin/rake webpacker:compile"
+      end
     end
   end
 
   task :watch => 'yarn:install' do
-    sh "cd builder && bin/webpack --watch"
+    Bundler.with_clean_env do
+      sh "cd builder && bin/webpack --watch"
+    end
   end
 
   namespace :yarn do