Cannot find module 'log4js'

After npm install, I tried to run the producer, I got this error:

```
Error: Cannot find module 'log4js'
at Function.Module._resolveFilename (module.js:325:15)
at Function.Module._load (module.js:276:25)
at Module.require (module.js:353:17)
at require (internal/module.js:12:17)
at Object. (/usr/src/app/samples/util/logger.js:18:14)
at Module._compile (module.js:409:26)
at Object.Module._extensions..js (module.js:416:10)
at Module.load (module.js:343:32)
at Function.Module._load (module.js:300:12)
at Module.require (module.js:353:17)
```

So, what's wrong?

Start with samples/util/logger.js at the require that raises the error, then inspect the project's npm dependency declarations and installation instructions. Reproduce the producer startup after npm install and verify that the log4js module resolves and the producer runs without this error.

Written by the indexing model from the issue text.