more conservative: only modify webpack's subclass of ProcessWrapper

lib/pipeline/webpackProcess.js

@@ -10,6 +10,11 @@ const path = require('path');
 let webpackProcess;
 
 class WebpackProcess extends ProcessWrapper {
+  constructor(options) {
+    super(options);
+    this.env = options.env;
+  }
+
   build(file, importsList, callback) {
     const self = this;
     let realCwd;

lib/process/processWrapper.js

@@ -17,7 +17,6 @@ class ProcessWrapper {
    */
   constructor(_options) {
     this.interceptLogs();
-    this.env = _options ? _options.env : 'development';
     this.events = new Events();
 
     this.pingParent();