Added error message when user tries to run tests without having the appropriate RubyMotion version available.

I ran into an issue where I couldn't run the specs in a newly cloned repo because I didn't have the correct RubyMotion version.

```shell
rake aborted!
LoadError: cannot load such file -- motion/project/template/ios
/Users/kvg/Code/app/ProMotion/Rakefile:3:in `<top (required)>'
(See full trace by running task with --trace)
```

This update shows the user a more helpful error message including a suggestion on how to fix the root cause.

```shell
Couldn't find RubyMotion 2.33. Run `sudo motion update --cache-version=2.33`.
```