correct pretender version is 0.4.2, not 0.4.3

The [commit message in the pretender repo says 0.4.3](https://github.com/trek/pretender/commit/0a802e8c544bae1ec8a1987d7586388d677bbd91), but in actuality the latest Pretender version was only bumped to 0.4.2.

I moved too fast with the [previous pr](https://github.com/rwjblue/ember-cli-pretender/pull/22), apologies.