Make iOS suite template work on Xcode 6

Apple has renamed the base iOS app templates in Xcode 6.  Since Xcode
helpfully ignores missing inherited templates, this change makes the
templates inherit from the old and new templates, which seems to work.

Also, prefix headers have been removed from Xcode 6 templates, so
ExampleSpec now imports Cedar itself so that it compiles (it probably
should have been doing this anyway).

Note that the prefix header files are still generated because targets
generated with Xcode 5 templates still depend on them, but they can be
safely deleted if created with Xcode 6.