update testem version, enables api proxy in testing

With the latest update to testem it's now possible to run the tests and proxy to the api stubs served by `ember serve` adding something like this

```json
  "proxies": {
    "/api": {
      "port": 4200,
      "host": "localhost"
    }
```
to `testem.json` 

if the work on  #1423 continues we won't need to have `ember serve` active during tests anymore

if interested see discussion in https://github.com/airportyh/testem/issues/218 and https://github.com/airportyh/testem/pull/391

couple of questions:
* shall we add this to the docs within this PR?
* shall we add tests? this is kind of external to ember-cli as all the behaviour is in testem
* shall this be tagged as ENHANCEMENT or INTERNAL?

thanks!