switch rvm versioning, make jshint happy, and add demo docs

I saw the demo directory and hit a couple of issues getting it going.

The latest version of `rvm` was unhappy (and kept complaining about "trust") with the `.rvmrc` file, but suggestions on the googles said that switching to the `.ruby-version` format would fix things.

I didn't want to force this on you as I wasn't sure it worked for everyone or maybe you're stuck on an older version of rvm, hence the pull request.

I also fixed up the indentation in the grunt file that jshint complained about when running the default `grunt` task and added some quick notes in the readme to explain how to get the demo working.

Feel free to edit/reject if some of this doesn't work for you.