Removes unnecessary env variable

Author: braposo

package.json

@@ -8,8 +8,8 @@
         "build": "yarn run build:commonjs && yarn run build:es && npm run build:umd && npm run build:umd:min",
         "build:es": "babel src -d es",
         "build:commonjs": "cross-env BABEL_ENV=commonjs babel src -d lib",
-        "build:umd": "cross-env BABEL_ENV=commonjs NODE_ENV=development webpack src/index.js -o dist/react-responsive-picture.js",
-        "build:umd:min": "cross-env BABEL_ENV=commonjs NODE_ENV=production webpack src/index.js -o dist/react-responsive-picture.min.js",
+        "build:umd": "cross-env NODE_ENV=development webpack src/index.js -o dist/react-responsive-picture.js",
+        "build:umd:min": "cross-env NODE_ENV=production webpack src/index.js -o dist/react-responsive-picture.min.js",
         "clean": "rimraf lib dist es",
         "dev": "yarn run clean && cross-env BABEL_ENV=commonjs babel src -d lib --watch",
         "lint": "eslint src/ --ext .js,.jsx",