The following worked for me (tested on react native 0.38 and 0.40):
npm install -g ios-deploy
# Run on a connected device, e.g. Max's iPhone:
react-native run-ios --device "Max's iPhone"
This is the proper way to do this according to fb. If you try to run just the run-ios command, you will find that the script recommends that you npm install -g ios-deploy when it comes to the install step after building.
While the documentation on the various commands that react-native offers is a little sketchy, it is worth doing as they suggest and going in to look at react-native/local-cli. There you can see all the commands available and the code that they run - you can thus work out what switches are available for undocumented commands.