add manual pretender shutdown to fix acceptance tests (#342)
add manual pretender shutdown to fix acceptance tests (#342)

no issue
- a [recent update](https://github.com/pretenderjs/pretender/pull/178) to Pretender contained a breaking change that throws an error when multiple pretender instances exist
- using mirage in acceptance tests was [triggering the error](https://github.com/samselikoff/ember-cli-mirage/issues/915)
acburdine/Ghost-Admin,TryGhost/Ghost-Admin,TryGhost/Ghost-Admin,kevinansfield/Ghost-Admin,JohnONolan/Ghost-Admin,kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,JohnONolan/Ghost-Admin