devscript: check ruby version before execution

When `developer/bin/list_running_app_ids` is run with `ruby < 1.9`, the following error will occur:

```bash
developer/bin/list_running_app_ids:58:in `load_apps': undefined method `capture3' for Open3:Module (NoMethodError)
	from developer/bin/list_running_app_ids:101
```

For Ruby-illterate people (such as myself for the most part), that error doesn't really describe how to fix the error (yes, a problem lies in the `open3` module, but most Cask authors, especially first time devscript users, would not know how to fix the issue or, even worse, not bother using the devscript/give up on making a Cask). This pull request places a small check at the beginning of execution to make sure the script is compatible. Now the error returned is:

```bash
You are currently using Ruby 1.8.7, but version 1.9 or above is required.
```