chore: ensure that build is done in NODE_ENV=production

Author: gajus

package.json

@@ -39,7 +39,7 @@
   },
   "scripts": {
     "add-assertions": "babel-node --presets es2015 ./bin/readme-assertions",
-    "build": "babel ./src --out-dir ./dist --copy-files --source-maps",
+    "build": "NODE_ENV=production babel ./src --out-dir ./dist --copy-files --source-maps",
     "generate-readme": "gitdown ./.README/README.md --output-file ./README.md && npm run readme-add-assertions",
     "lint": "eslint ./src ./test",
     "test": "mocha --compilers js:babel-register && npm run lint"