Minor config fixes - *some browserstack property names changed*

If you are using platform and platformVersion to refer to browserstack, you will need to note 
```
"browserStackPlatform" : "Windows" //only if you are using a remote driver for Browserstack
"browserStackPlatformVersion" : "7" //only if you are using a remote driver for Browserstack
```
The reason is the old properties clashed with the saucelabs ones, but didn't have the same values meaning runtime errors when switching.  Also the default is removed so if you're using a local grid it won't try to set it.
@jamesoram @Shakor @istvanpamer + others feel free to comment
no but if you're using sauce labs presumably you'd be reading their documentation to find the valid values - I've made the default work as it just doesn't set it in the capabilities.  which particular bit were you worried about?