Updating readme to include a note about removing .gem directory

Adding a note under the installation section to clarify that you should remove the `.gem` directory from your home directory.

I found it difficult to debug and this would have saved me a lot of time. After following the current installation instructions (and having removed rvm) I could run `ruby -v` and get the rbenv ruby version I'd set with `rbenv global`, run `gem install rails` and see rails get installed, but running `rails new project` would tell me I needed to install the gem.

It wasn't until I found this [StackOverflow answer](http://stackoverflow.com/a/11146496/836205) with the random note about removing `~/.gem` that I could successfully use the rails command.

I think this is worth calling out in the README as it doesn't seem to be a particularly rare issue. I don't know enough about rbenv's shims and rubygems to know if there's an underlying issue that should be fixed.