Improve cucumber tests

We checked that the rake spec tasks exit 0 when the tests fail. 

We noticed that someone had updated the iOS rakefile so that it would work with xcodebuild from XCode 5, where `RunUnitTests` is no longer available. This change had not been applied to the OS X test bundle Rakefile template, so we fixed that. 

We've left DRYing up the cucumber tests and Rakefiles as an exercise for the reader. If we accumulate any more cucumber tests, it might be nice to write some infrastructure around that.