Fix to NodeJS link and alteration to OroRequirements.php

Detect whether a js engine is installed rather then just "node"
If the settings in the config.yml are altered to the following it will check the version of that js_engine.
Update to the README.md to show below yaml for Ubuntu installs 

```
oro_require_js:
    js_engine:  "nodejs"
```

This allows the checks to pass if an alternate js_engine is installed