Use cross-env to enable windows build

Author: acailly

package.json

@@ -29,6 +29,7 @@
     "canvas": "^1.6.2",
     "chai": "^3.5.0",
     "chart.js": "^2.3.0",
+    "cross-env": "^5.0.0",
     "debug": "^2.4.1",
     "enzyme": "^2.6.0",
     "eslint": "^1.6.0",
@@ -54,11 +55,11 @@
     "chart.js": "global:Chart"
   },
   "scripts": {
-    "build": "gulp clean && NODE_ENV=production gulp build",
+    "build": "gulp clean && cross-env NODE_ENV=production gulp build",
     "examples": "gulp dev:server",
     "lint": "eslint ./; true",
-    "publish:site": "NODE_ENV=production gulp publish:examples",
-    "release": "NODE_ENV=production gulp release",
+    "publish:site": "cross-env NODE_ENV=production gulp publish:examples",
+    "release": "cross-env NODE_ENV=production gulp release",
     "start": "gulp dev",
     "test": "mocha test/config/setup.js test/__tests__/**/*",
     "watch": "gulp watch:lib",