should be able to launch any simulator

***Do not merge.***

## motivation

The Launcher `default_args` method was not respecting the `DEVICE_TARGET` env bar when launching the simulator; it was always setting

```
args[:device_target] = 'simulator'
```
even when `DEVICE_TARGET` set to the another simulator.

As a result, the default 'iPhone 4in iOS 7' was _always_ being launched.

## todo

- [x] needs briar (automated) tests - @jmoody [jenkins](http://ljs-jenkins.ngrok.com/job/briar-pr/10/)
- [x] needs review by @krukow 
- [ ] needs a Xamarin IDE user to confirm

## tests

```
Finished in 5 minutes 34 seconds
46 examples, 0 failures
```