Fix variable name

lib/install/config/babel.config.js

@@ -10,7 +10,7 @@ module.exports = function(api) {
       'Please specify a valid `NODE_ENV` or ' +
         '`BABEL_ENV` environment variables. Valid values are "development", ' +
         '"test", and "production". Instead, received: ' +
-        JSON.stringify(env) +
+        JSON.stringify(currentEnv) +
         '.'
     )
   }