Add relative paths to package.path when running in simulator to make the example work straight out of the box

This adds the parent directory (spine-runtimes) to package.path allowing the example to work straight out of the box - currently it just throws an error and at least someone on the Corona forums was confused by it. 

However, I'm not sure if you really want to do this - feel free to not merge! - as it hides the reason why it's not working a bit deeper which probably will cause some problems later on for the inexperienced user or possibly just add some documentation to the example. Another option could be to use git itself to somehow include copy of spine-lua in the spine-corona directory? Not sure what the best way to do that would be, though.