Fix server props port

Under `nemoData` when  `targetServer:localhost` and ```serverProps``` is absent then the original logic skips the process of starting the selenium server. Now `serverUrl` defaults to `localhost`. This causes an error when building the driver with following error,

```exception
  Uncaught Error: Invalid server URL: localhost
      at new HttpClient (/Users/awesometest/node_modules/nemo/node_modules/selenium-webdriver/http/index.js:37:11)
......more error here..
```
We need to handle this gracefully and inform the user that he/she needs to minimally provide port to run local selenium server under serverProps (as per the documentation). 

The documentation says "Leave (targetServer) unset if you are using chrome or phantomjs on your local machine". However we check whether targetServer is local or not without checking it's absence. I therefore defaulted it to `localhost`
I'm trying to avoid throwing errors around configuration. Because it assumes a lot about how people may be trying to use selenium-webdriver. Already (for the case of using Perfecto Mobile) one of the thrown errors was making it impossible for people to use Nemo. So, the preference going forward will be to use the debug/error logging (currently in the develop branch).