Integrate ActiveJob / DeliverLater / GlobalID with Rails

WIP.
Is this actually necessary?  You can get the currently loaded version of a gem from RubyGems:

```
irb(main):001:0> require 'nokogiri'
=> true
irb(main):002:0> Gem.loaded_specs['nokogiri'].version
=> #<Gem::Version "1.6.3.1">
irb(main):003:0>
```

Also, what does anyone use this for?