Updated ENV for webpack-dev-server 4.0

Author: David Heinemeier Hansson

lib/webpacker/dev_server_runner.rb

@@ -65,7 +65,7 @@ def detect_port!
       def execute_cmd
         env = Webpacker::Compiler.env
         env["WEBPACKER_CONFIG"] = @webpacker_config
-        env["WEBPACK_DEV_SERVER"] = "true"
+        env["WEBPACK_SERVE"] = "true"
 
         cmd = if node_modules_bin_exist?
           ["#{@node_modules_bin_path}/webpack", "serve"]