Make gem loading work when GEM_PATH isn't defined

Hi, your setup is impressive, thanks! I am looking into replicating/integrating it with my current VIM settings etc. 

I had switched from RVM to rbenv months ago but I have now switched back so I could use your simple installation procedure without having to figure out how to make it work with rbenv. I am not sure why though, on my system GEM_PATH isn't defined after installing RVM and refreshing my shell (ZSH).

I have found though that 

```ruby
`rvm $(rvm current) do gem env path`.chop
```

yields the same return (seems to work both with and without an .rvmrc file in the project's folder); so I have made a little change to use this command if GEM_PATH isn't defined like in my case.

Please let me know if otherwise there is a reason -and a fix- why this variable isn't automatically defined in some scenarios.

Thanks!