run ./bin/webpack with --json flag in webpacker:compile (#199)

this flag was removed in #153, but without it the error handling block
throws a `JSON::ParserError` and dumps the webpack output

lib/tasks/webpacker/compile.rake

@@ -5,7 +5,7 @@ namespace :webpacker do
   desc "Compile javascript packs using webpack for production with digests"
   task compile: ["webpacker:verify_install", :environment] do
     puts "Compiling webpacker assets 🎉"
-    result = `NODE_ENV=production ./bin/webpack`
+    result = `NODE_ENV=production ./bin/webpack --json`
 
     unless $?.success?
       puts JSON.parse(result)["errors"]