Xcode 6 support

~~The unit tests don't currently build, because Specta/Expecta haven't yet been updated for Xcode 6.~~
This made me curious, so I went to the docs to find what Apple has to say about the matter:

> The bundle identifier string identifies your application to the system. This string must be a uniform type identifier (UTI) that contains only alphanumeric (A-Z,a-z,0-9), hyphen (-), and period (.) characters. The string should also be in reverse-DNS format. For example, if your company’s domain is Ajax.com and you create an application named Hello, you could assign the string com.Ajax.Hello as your application’s bundle identifier.
>
> The bundle identifier is used in validating the application signature.

So apparently they do suggest using a domain. However, there are still a couple of reasons I'd like to avoid that:

 1. GitHub is “just” the home of ReactiveCocoa—it's not the owner of it. Incorporating `github` into the bundle ID seems to tacitly imply that there might be different ReactiveCocoas that differ in where they're hosted.
 1. What if ReactiveCocoa moved somewhere else in the future? Obviously, we work at GitHub, so it's unlikely to be another repository host, but it could be something as simple as making reactivecocoa.io more of a “thing”—should that really necessitate changing the bundle ID?

I think of bundle IDs as a fairly immutable unique identifier for a project. I don't like specifying something that could change over time.