Fixed a few issues with the README file.

I just installed this and ran into some small issues with the README file. And I wanted to make it clear that redis is a dependency (though maybe there are others that were already installed on my computer and that should also be mentioned? )

@cvan r?
I'd be more inclined to not explain how to install those dependencies, but only to list them. It's up to the person who's installing this to use his or her favourite search engine to figure it, imho. 

Is PostgreSQL a dependency yet?