fix #69 lesspass/pure build failed on bash in window

package.json

@@ -6,7 +6,7 @@
   "repository": "lesspass/pure",
   "scripts": {
     "dev": "npm install && webpack-dev-server --inline --hot --host 0.0.0.0",
-    "build": "rm -rf dist && NODE_ENV=production webpack --display-error-details --progress --hide-modules",
+    "build": "rm -rf dist && cross-env NODE_ENV=production webpack --display-error-details --progress --hide-modules",
     "test": "ava test --compilers js:babel-register",
     "prepublish": "npm test && npm run build"
   },
@@ -32,6 +32,7 @@
     "babel-preset-stage-2": "^6.17.0",
     "babel-register": "^6.16.3",
     "babel-runtime": "^6.11.6",
+    "cross-env": "^3.1.3",
     "css-loader": "^0.25.0",
     "extract-text-webpack-plugin": "2.0.0-beta.4",
     "file-loader": "^0.9.0",